Polymorphism of the Transition Metal Oxidotellurates NiTeO₄ and CuTe₂O₅

Abstract

As part of crystal growth experiments on transition metal oxidotellurates using chemical vapor transport reactions or hydrothermal conditions, single crystals of Ni^IITe^VIO₄ and Cu^IITe^IV₂O₅ were obtained for the first time in the form of new modifications, as revealed by crystal structure determinations from X-ray data. In the course of these investigations, the crystal structure model of the only phase of Ni^IITe^VIO₄ reported so far (from now on named α-) was corrected. Both α-(space group P2₁/c, Z = 2) and the new β-polymorph of Ni^IITe^VIO₄ (space group I4₁/a, Z = 8) can be considered derivatives (hettotypes) of the rutile structure (aristotype), as shown by detailed symmetry relationships. For CuTe₂O₅ also, only one crystalline phase has been described so far (from now on named α-) that corresponds to the mineral rajite (space group P2₁/c, Z = 2). Its anion comprises two different trigonal-pyramidal TeO₃ groups linked through corner-sharing into a ditellurite unit. The anion part of the new β-CuTe₂O₅ modification (space group P2₁/c, Z = 2), likewise, comprises two Te^IV atoms but is more complex. Here, one Te^IV atom exhibits a coordination number of 4 and is part of a ${}_{\propto }{}^1[$TeO_2/2O_2/1] chain, and the other has a coordination number of 5 and is part of a ${}_{\propto }{}^1[$TeO_2/2O_3/1]₂ dimer. The two types of anions are linked into a tri-periodic framework where both Te^IV atoms are stereochemically active. The α- and β-CuTe₂O₅ modifications show no closer structural relationship, which is also reflected in their clearly different Raman spectra. Data mining for knowledge discovery in a structure database reveals that polymorphism is a rather common phenomenon for the family of inorganic oxidotellurates.

1. Introduction

The large family of oxidotellurates has a very broad spectrum of compositions and crystal structures. This is partly due to the fact that tellurium can be tetravalent or hexavalent in these compounds, and mixed-valent Te^IV/VI compounds also occur. The distinct ability to condense the corresponding anions (commonly referred to as “tellurites” for oxidotellurates(IV) and “tellurates” for oxidotellurates(VI)) also contributes to the structural diversity. A recent review of the crystal chemistry of this family of compounds summarized and categorized their peculiar structural features [1].

First-row transition metal oxidotellurates(VI) with composition MTeO₄ (M = Co, Ni, Cu, Zn) exhibit interesting structural and physical properties: CoTeO₄ adopts the monoclinic dirutile structure type and is a wide-bandgap material [2]; NiTeO₄, structurally determined from powder synchrotron data, is isotypic with CoTeO₄ and shows a robust electrochemical activity against Li metal with a high reversible specific capacity [3]; CuTeO₄ is reported to crystallize with an order/disorder structure [4] and shows quasi-one-dimensional, antiferromagnetic correlations at high temperature [5]; ZnTeO₄ is assumed to have great second-harmonic generation coefficients and a large birefringence, as predicted by first principles techniques on the basis of a theoretically deduced crystal structure [6].

In order to understand and accurately calculate the physical properties mentioned above, a reliable crystal structure model is of utmost importance. Furthermore, in the context of structure–property relationships, it is desirable to have highly crystalline and single-phase material available for the measurement of other physical properties related to long-range ordering and correlated phenomena. For these reasons, single-crystal growth experiments were carried out for the two phases NiTeO₄ and CuTeO₄. In the case of NiTeO₄, the existing structure model [3] is highly questionable. For crystal growth of this compound, chemical vapor transport reactions (CVTR) [7] were applied, the use of which had already been shown to be successful with the closely related compound CoTeO₄ [2]. For CuTeO₄, single-crystal growth was primarily intended to investigate the reported differences in the crystal structure models obtained from single-crystal diffraction [4] and powder diffraction [5] data. For crystal growth of CuTeO₄, the hydrothermal synthesis method [8] was chosen, but under considerably different pressure and temperature conditions than in the original report (650 °C, 2000 bar [4]) or under slightly different conditions than applied for preparation of polycrystalline material (210 °C, autogenous pressure, filling degree of the container ≈ 50% [5]).

In the course of these experiments, two crystalline modifications of NiTeO₄ (one known and one new) as well as a new crystalline modification of the unexpected product CuTe₂O₅ were obtained. The results are reported and discussed here.

2. Materials and Methods

2.1. Crystal Growth Experiments

NiTeO₄. Polycrystalline NiTeO₄ was prepared by heating a stoichiometric 1:1 mixture of Ni(NO₃)₂·6H₂O and Te(OH)₆ in a porcelain crucible at 550 °C for one week under multiple intermediate grinding. Powder X-ray diffraction measurement revealed the product to be single-phase according to previously reported data [3]. About 0.5 g of the resulting ochre-colored powder was filled into a silica glass ampoule (volume ≈ 20 cm³) together with 50 mg PtCl₂ and sealed under vacuum. The ampoule was heated in a horizontal two-zone furnace for three weeks, applying a temperature gradient of 640 °C (source region) → 580 °C (sink region). Under these conditions, PtCl₂ decomposes and the released chlorine gas acts as a transport agent [7,9]. At the end of the experiment, three different crystal types were present in the sink zone, which were identified optically under a microscope and by subsequent single-crystal X-ray diffraction: yellow block-like Ni^IITe^IV₂O₅ crystals [10] as the main phase, brownish-orange plates of known dirutile-type NiTeO₄ [3] (subsequently referred to as α-NiTeO₄) as a minor phase, and very few brownish-green plate-like crystals of a new modification of NiTeO₄ (subsequently referred to as β-NiTeO₄).

CuTeO₄. Cu(NO₃)₂·3H₂O (0.284 g), Te(OH)₆ (0.270 g), and KOH (0.132 g) were mixed in the molar ratio 1:1:2 and placed in a Teflon container with an inner volume of ≈6 mL to which water was added (filling degree ≈ 70%). Then the mixture was mechanically stirred for 30 min, and the container was sealed and placed in a steel autoclave at 210 °C for one week under autogenous pressure. Very few bottle-green plate-like crystals of a new modification of CuTe₂O₅ (subsequently referred to as β-CuTe₂O₅) were obtained as the only solid reaction product. These crystals were used for the single-crystal X-ray diffraction and Raman spectroscopic measurements.

2.2. X-Ray Diffraction Measurements and Crystal Structure Analysis

Powder X-ray diffraction (PXRD) measurements were performed on a PANalytical X’Pert II Pro type PW 3040/60 diffractometer (Malvern Panalytical, Malvern, Worshestershire, UK) using Cu-K_α1,2-radiation and an X’Celerator detector.

Single-crystal X-ray diffraction data were recorded on a Bruker Kappa Apex-II single-crystal diffractometer (Bruker AXS, Madison, WI, USA) equipped with a CCD detector and using graphite-monochromatized Mo-K_α-radiation. Instrument software packages (Apex-4 and Saint [11]) were used for optimized measurement strategies (>99% completeness at θ_max) and for data reduction; absorption effects were corrected with Sadabs [12]. The crystal structures were solved with Shelxt [13], refined with Shelxl [14] and graphically represented with Atoms [15]. For the crystal structure of α-NiTeO₄, atom labels and coordinates were assigned in accordance with the previously reported isotypic crystal structure of CoTeO₄ [2].

Crystal structure and refinement data are compiled in

Table 1. Data collection and refinement details.

Formula		α-NiTeO4	β-NiTeO4	β-CuTe2O5
Temperature/°C		23	23	23
Mr		250.31	250.31	398.74
Crystal size/mm3		0.08 × 0.07 × 0.01	0.18 × 0.10 × 0.02	0.18 × 0.10 × 0.04
Crystal color, form		brownish-orange, plate	brownish-green, plate	green, plate
Crystal system		monoclinic	tetragonal	monoclinic
Space group, No.		P21/c, 14	I41/a, 88	P21/c, 14
a/Å		5.5285(6)	9.2627(5)	6.6599(4)
b/Å		4.6500(5)	9.2627(5)	7.6233(4)
c/Å		5.5449(6)	6.0988(4)	8.8413(5)
α/°		90	90	90
β/°		113.120(2)	90	92.6910(10)
γ/°		90	90	90
V/Å3		131.10(2)	523.26(7)	448.38(4)
Z		2	8	4
Dcalcd, /g cm–3		6.341	6.355	5.907
μ(MoKα), mm–1		18.093	18.132	17.541
Theta range/°		3.996–41.802	3.110–43.921	3.602–32.778
hkl range		−10 → 9	−18 → 17	−9 → 10
		−8 → 8	−18 → 18	−11 → 11
		−10 → 10	−11 → 11	−12 → 12
Reflections measured		5409	17891	7179
Reflections unique		943	1011	1580
Reflections [I > 2σ(I)]		694	703	1573
Rint		0.038	0.064	0.029
Extinction coefficient (Shelx)		0.0319(13)	-	0.0128(3)
Parameters refined		33	33	74
R(F2)/wR(F2) [I > 2σ(I)]		0.0181/0.0343	0.0246/0.0604	0.0160/0.0381
R(F2)/wR(F2) [all reflections]		0.0326/0.0378	0.0480/0.0680	0.0161/0.0382
GooF (F2)		1.030	0.984	1.197
Δρfin (max/min)/e− Å–3		1.569/−1.718	1.041/−1.107	0.862/−0.874
CSD-number		2418621	2418623	2418622

. Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service: https://www.ccdc.cam.ac.uk/structures/ by quoting the deposition numbers specified at the end of Table 1.

Bond valence sums (BVS) [16] were calculated using the bond valence parameters provided by Brese and O’Keeffe [17] for Ni–O and Cu–O bonds. For the Te–O bonds, the revised bond valence parameters by Mills and Christy [18] were used.

2.3. Raman Spectroscopy

Raman spectra of α- and β-CuTe₂O₅ were recorded using an RM 1000 micro-Raman system (Renishaw plc, Wolton-under-Edge, UK) in the range from 4000 to 40 cm⁻¹ on randomly oriented single crystals. The 488 nm excitation line of a 20 mW Ar laser was focused with a 50×/0.75 objective lens slightly below the sample surface. Raman intensities were collected with a thermo-electrically cooled CCD array detector. The resolution of the system was 5–6 cm⁻¹, and the wavenumber accuracy was ±1 cm⁻¹, both calibrated with the Rayleigh line and the 520.5 cm⁻¹ line of a Si standard.

3. Results and Discussion

3.1. Crystal Growth

The desired target compounds NiTeO₄ and CuTeO₄ could not be obtained as single-phase materials under the selected experimental conditions.

During CVTR of NiTeO₄, a partial reduction of the Te^VI-containing educt occurred under the formation of the Te^IV-containing phase NiTe₂O₅ as the main product in the sink region, whereas the expected phase NiTeO₄ was obtained only as a minor product both in its already known monoclinic dirutile-type α-phase and in a previously unknown tetragonal form, the β-phase. The remaining material in the source region consisted mainly of Ni₃TeO₆ [19] as revealed by PXRD measurements; some additional reflections were present that could not be assigned to any known phase. Possible equilibrium reactions for the gas phase transport of NiTeO₄ are:

NiTeO₄(s) + 2Cl₂(g) ⇌ NiCl₂(g) + TeOCl₂(g) +1.5O₂(g)

(1)

NiTeO₄(s) + 2Cl₂(g) ⇌ NiCl₂(g) + TeCl₂(g) + 2O₂(g)

(2)

The simultaneous occurrence of NiTe₂O₅ and NiTeO₄ in the sink region indicates a nonstationary transport behavior [20], probably caused by an incongruent dissolution of the solid in the source region under the consideration of several (other) independent equilibria, e.g., (3) and (4). As noted in [20], then “the targeted and possibly pure phase deposition of a substance notably depends on the composition of the initial solid, the transport agent, and the transport conditions, as well as the duration of the transport”.

3NiTeO₄(s) ⇌ Ni₃TeO₆(s)+ 2TeO₂(s) + O₂(g)

(3)

2NiTeO₄(s) ⇌ NiTe₂O₅(s)+ NiO(s) + O₂(g)

(4)

NiTe₂O₅(s) + 3Cl₂(g) ⇌ NiCl₂(g) + 2TeOCl₂(g) +1.5O₂(g)

(5)

The oxygen partial pressure plays a crucial role in the CVTR of an oxide material, as can be seen in Equations (1) and (2) in the case of NiTeO₄. If the oxygen partial pressure is higher than the dissociation pressure of the employed oxide material, the solid will be deposited in the source region, whereas a product with a lower oxygen content will be deposited if the partial pressure of oxygen is below the dissociation pressure of the employed oxide material [20]. Under the selected CVTR conditions for NiTeO₄, the oxygen partial pressure appears to lie just between these limits. It should be noted that CVTR of polycrystalline CoTeO₄ using TeCl₄ instead of Cl₂ as a transport agent under otherwise identical conditions, as described in Section 2.1, led to large, single-phase dirutile-type CoTeO₄ crystals [2], which are isotypic with α-NiTeO₄.

Similarly, under the given hydrothermal conditions for the intended crystal growth of CuTeO₄, Te^VI was reduced to Te^IV, resulting in the formation of the new β-CuTe₂O₅ modification. The elimination of oxygen from the employed telluric acid (or more realistically an oxidotellurate(VI) anion formed under the given basic conditions) is decisive here. This assumption is supported by the information provided in [5] where it is stated that CuTeO₄ was only obtained hydrothermally by the addition of hydrogen peroxide, which prevents the tendency to release oxygen under the reduction of Te^VI to Te^IV.

On the other hand, hydrothermal experiments for the targeted synthesis of β-CuTe₂O₅ by using stoichiometric amounts of Cu:Te in a ratio of 1:2 and K₂Te^IVO₃ as the tellurium source under otherwise identical conditions, as described in Section 2.1, led in all cases to the formation of the known modification [21] (subsequently referred to as α-CuTe₂O₅), which is also known as the mineral rajite [22]. Crystals of α-CuTe₂O₅ obtained this way were identified by single-crystal X-ray diffraction and were used for the subsequent Raman spectroscopic measurement.

3.2. Structure Description of α-NiTeO₄

The crystal structure of the known polymorph α-NiTeO₄ has originally been determined on the basis of laboratory PXRD data, with the Ni and Te atoms equally occupying the same general site (multiplicity 4, Wyckoff letter e) of space group P2₁/n [23]. In a recent powder synchrotron study, it was shown that the original model was incorrect in this regard, and next to two O positions at general sites, two distinct positions associated with an inversion center were assigned for Ni (2 b) and Te (2 c) [3]. However, the current redetermination of α-NiTeO₄ based on single-crystal X-ray data clearly showed that this assignment is also incorrect and that the two positions must be swapped. This is confirmed by the calculation of the global instability index (GII) [24]. As guide values, for stable crystal structures, GII is given as <0.1 valence units, and for unstable crystal structures, as GII > 0.2 valence units [25]. The calculation of GII using the program bVsoft [26] with the reported structure data [3] reveals a very high value of 0.62 valence units, which clearly indicates a highly questionable crystal structure model. Taking into account a shift by ½ 0 0 in the unit cell relative to the positions given in [3], Ni is located on position 2 a and Te on position 2 d in the corrected model presented here. The latter shows a significant improvement in terms of the accuracy of bond lengths, as a comparison in

Table 2. α-NiTeO₄ and β-NiTeO₄. Selected bond lengths/Å, angles/°, as well as bond valence sums (BVS)/v.u.

α-NiTeO4 (the italicized figures in brackets refer to a previous but incorrect model [3])
Ni1–O2 i	2.0371(16) [1.995]	Te1–O2 vi	1.8615(16) [1.974]
Ni1–O2 ii	2.0371(16)	Te1–O2 ii	1.8615(16)
Ni1–O2 iii	2.0689(15) [1.998]	Te1–O1	1.9617(16) [2.051]
Ni1–O2 iv	2.0689(15)	Te1–O1 vii	1.9618(15)
Ni1–O1	2.0691(16) [1.837]	Te1–O1vi	1.9721(15) [2.079]
Ni1–O1 v	2.0691(16)	Te1–O1 ii	1.9721(15)
BVS: Ni1 2.01 [2.68], Te1 5.91 [4.71], O1 2.17 (CN 3; 1 Ni, 2 Te) [1.69], O2 1.79 (CN 3; 2 Ni, 1 Te) [1.99].
Symmetry codes: (i) x−1, −y+1/2, z−1/2; (ii) −x+1, y−1/2, −z+1/2; (iii) −x+1, −y, −z; (iv) x−1, y, z; (v) −x, −y, −z; (vi) x, −y+1/2, z+1/2; (vii) −x+1, −y, −z+1.
β-NiTeO4
Ni1–O1 i	2.033(3)	Te1–O1 i	1.861(3)
Ni1–O1 ii	2.033(3)	Te1–O1 viii	1.861(3)
Ni1–O2 iii	2.068(3)	Te1–O2 iii	1.955(3)
Ni1–O2 iv	2.068(3)	Te1–O2 ix	1.955(3)
Ni1–O1 v	2.070(2)	Te1–O2 x	1.970(2)
Ni1–O1 vi	2.070(2)	Te1–O2 xi	1.970(2)
BVS: Ni1 2.02, Te1 5.94, O1 (CN 3; 2 Ni, 1 Te), O2 2.18 (CN 3; 1 Ni, 2 Te).
Symmetry codes: (i) y−1/4, −x+1/4, −z+1/4; (ii) −y+1/4, x−1/4, z−1/4; (iii) −y+1/4, x−3/4, −z+1/4; (iv) y−1/4, −x+3/4, z−1/4; (v) −x, −y+1/2, z; (vi) x, y−1/2, −z; (vii) x, y, z−1; (viii) −y+1/4, x−1/4, z+3/4; (ix) y−1/4, −x+3/4, z+3/4; (x) −x+1/2, −y+1/2, −z+1/2; (xi) x−1/2, y−1/2, z+1/2.

shows. Correspondingly, much more satisfactory BVS values close to the expected values of 2, 6, and 2 for Ni, Te, and O are obtained.

α-NiTeO₄ crystallizes in a dirutile-type structure in the monoclinic space group P2₁/c. In the crystal structure, the Ni1 atom (site symmetry $\overline{1}$) is octahedrally surrounded by O atoms with three pairs of similar Ni–O bond lengths (Table 2; mean value 2.058 Å), in good agreement with the literature value for six-coordinate Ni^II (2.070(54) Å [27]). The Te1 atom (site symmetry $\overline{1}$) is in oxidation state +VI and shows the common octahedral coordination [1] with bond lengths in the characteristic range (mean 1.932 Å, literature value 1.923(39) Å [28]). Edge-sharing occurs between alternating NiO₆ and TeO₆ octahedra, resulting in the formation of strands that extend parallel to [101] (Figure 1a). Each strand is surrounded by four symmetry-equivalent neighboring strands, each of which is arranged at an angle of approximately 90° relative to the central strand along the [101] direction, and connected to the central strand via common corners (Figure 1b).

3.3. Structure Description of β-NiTeO₄

The new polymorph β-NiTeO₄ crystallizes in the tetragonal space group I4₁/a. The asymmetric unit comprises one Ni^II atom with site symmetry $\overline{1}$ (8 c), one Te^VI atom with site symmetry $\overline{1}$ (8 d), and two O atoms located in general sites (16 f). Like in the α-form, both Ni and Te sites in β-NiTeO₄ are octahedrally surrounded by O atoms, and the individual bond lengths (Table 2) in the two modifications are nearly identical, with mean Ni–O and Te–O bond lengths of 2.057 Å and 1.929 Å, respectively, for the β-modification. Likewise, the connection of the octahedral building units is similar in the two modifications. In the crystal structure of β-NiTeO₄, the strands made of edge-sharing alternating NiO₆ and TeO₆ octahedra extend along [001] (Figure 2a), and neighboring strands are, in turn, twisted by 90° against this strand and connected via common corners (Figure 2b).

3.4. Structure Relation of α- and β-NiTeO₄ to Rutile

Both modifications crystallize in hettotypes of the rutile aristotype and can be described as a twofold (α-NiTeO₄) or eightfold (β-NiTeO₄) cation-ordered superstructure. As in rutile itself [29], the characteristic connection patterns, namely, strands of edge-linked octahedra that are fused to neighboring strands via common corners, can, therefore, be found in both polymorphs. Likewise, the typical environments of the oxygen atoms in the rutile structure are also observed in both polymorphs, i.e., each oxygen atom is bound in a (distorted) trigonal-planar manner to three positively charged atoms. The symmetry relationships [30] between the rutile aristotype and the two hettotypes are graphically represented in Figure 3 in the form of a Bärnighausen tree [31]. Baur has described the structural relationships of rutile and its known derivatives in detail [29] and has already mentioned the symmetry descent from rutile to the monoclinic dirutile type (referred to there as the CUU type), which is adapted by α-NiTeO₄ and described in detail for isotypic CoTeO₄ [2]. However, the crystal structure of the β-modification has not yet been described and, therefore, represents a new derivative of the rutile structure type. A direct symmetry derivation of β-NiTeO₄ from rutile is not possible and proceeds via two hypothetical intermediate stages for which no representatives are yet known. In a first step, a translationengleiche (t) symmetry reduction of index 2 takes place, followed by a klassengleiche (k) symmetry reduction of index 2 under doubling of the unit cell. In the last step, another klassengleiche symmetry reduction k2 occurs under the ordering of the hypothetical M site into two cationic sites (Ni and Te) and a splitting of the hypothetical X site into two O sites. Due to the resulting I-centering, the cell volume is quadrupled.

The simultaneous occurrence of crystals of α- and β-NiTeO₄ in the transport ampoule suggests that the energetic differences between the two polymorphic forms are small. However, the synthesis of polycrystalline NiTeO₄ by solid-state reactions has always been reported to yield the monoclinic α-form [3,23], indicating that this may be the thermodynamically stable modification. Based on symmetry relationships, a direct transition from the tetragonal β-form, which has a larger crystal volume, to the monoclinic α-form, which has a smaller volume, is not possible. It is, therefore, likely that any phase transition that occurs between the two phases is of a reconstructive nature.

3.5. Structure Description of β-CuTe₂O₅

The asymmetric unit of β-CuTe₂O₅ consists of one formula unit, with all atoms on general positions (multiplicity 4, Wyckoff letter e) of space group P2₁/c. Selected bond lengths and angles are collated in

Table 3. β-CuTe₂O₅. Selected bond lengths/Å, angles/°, as well as bond valence sums (BVS)/v.u.

Cu1–O3	1.9523(18)	O3–Te1–O5	98.32(6)
Cu1–O5	1.9626(18)	O3–Te1–O4	94.73(6)
Cu1–O2 i	1.9694(18)	O5–Te1–O4	98.82(5)
Cu1–O1 ii	1.9950(18)	O2–Te1–O5	92.16(8)
Cu1–O5 iii	2.4727(19)	O2–Te1–O1	96.84(8)
Cu1–O3 iv	2.5635(19)	O5–Te1–O1	93.96(8)
Te1–O2	1.8979(18)	O2–Te1–O5 v	71.42(7)
Te1–O5	1.9010(17)	O5–Te1–O5 v	160.50(5)
Te1–O1	1.9067(18)	O1–Te1–O5 v	78.14(7)
Te1–O5 v	2.4953(18)	O3–Te2–O4	93.31(8)
Te2–O3	1.8585(17)	O3–Te2–O4 vi	93.25(8)
Te2–O4	1.9635(17)	O4–Te2–O4 vi	77.75(8)
Te2–O4 vi	2.0413(17)	O3–Te2–O1 vii	78.88(7)
Te2–O1 vii	2.3041(17)	O4–Te2–O1 vii	85.69(7)
Te2–O2 i	2.4026(18)	O4vi–Te2–O1 vii	161.26(7)
		O3–Te2–O2 i	73.87(7)
		O4–Te2–O2 i	156.08(7)
		O4vi–Te2–O2 i	82.81(7)
		O1vii–Te2–O2 i	110.71(7)

BVS: Cu1 2.03, Te1 3.73 (CN 3+1), Te2 3.87 (CN 3+2), O1 2.01 (CN 3; 1 Cu, 2 Te,), O2 1.96 (CN 3; 1 Cu, 2 Te), O3 1.85 (CN 3; 2 Cu, 1 Te), O4 1.82 (CN 2; 2 Te), O5 1.84 (CN 4; 2 Cu, 2 Te). Symmetry codes: (i) −x, y+1/2, −z+1/2; (ii) x, −y+1/2, z−1/2; (iii) −x, −y+1, −z; (iv) −x+1, −y+1, −z; (v) −x, y−1/2, −z+1/2; (vi) −x+1, −y+2, −z; (vii) −x+1, y+1/2, −z+1/2.

.

The copper(II) atom shows the characteristic [4+2] Jahn–Teller distortion [32] with four short equatorial Cu–O bonds between 1.9523(18) and 1.9950(18) Å and two longer axial bonds of 2.4727(19) and 2.5635(19) Å, leading to a tetragonally distorted CuO₆ octahedron. The mean Cu–O bond length (2.153 Å) corresponds almost perfectly to the literature value of 2.130(232) Å [27]. Individual CuO₆ octahedra are linked through edge-sharing into ${}_{\propto }{}^1[$CuO_4/2O_2/1] chains extending parallel to [100] whereby the long bonds are those to the edge-sharing atoms (Figure 4a).

The coordination environments of the two Te^IV atoms are clearly divided into two parts. If only Te–O bond lengths smaller than 2.1 Å are taken into account as being part of the inner coordination sphere, both Te^IV atoms have three nearest oxygen neighbors in the form of the frequently observed TeO₃ trigonal pyramid [1] as a coordination polyhedron: Te1 shows an almost equidistant Te–O bond length distribution (range 1.8979(18)–1.9067(18) Å), while Te2 shows clear differences in these bond lengths, with one very short (1.8585(17) Å), one intermediate (1.9635(17) Å), and one longer bond (2.0413(17) Å), which, for the latter, can be related to the formation of centrosymmetric edge-sharing dimers, with the shortest bond being that to the terminal O atom. The outer coordination spheres of the two Te^IV atoms include more remote O atoms within a threshold of 2.5 Å, and satisfactory BVS parameters for the two Te atoms can only be obtained if these longer bonds partake in the calculations (Table 3). Then Te1 is surrounded by one additional O atom at a distance of 2.4953(18) Å, with the resulting polyhedron having the shape of a distorted bisphenoid. Although the mean bond length of 2.050 Å for the Te1O₄ polyhedron is greater than the literature value (1.984(123) Å) [28]), the deviation is within the single standard deviation.

The Te1O₄ polyhedra share corner atoms (O5) to build up ${}_{\propto }{}^1[$TeO_2/2O_2/1] chains propagating parallel to [010] whereby short and long bond lengths to the corner atoms alternate along the chain (Figure 4b). On the other hand, Te2 has even two additional bonding O atoms in the outer coordination sphere with distances at 2.3041(17) and 2.4026(18) Å, resulting in a distorted square pyramid as coordination polyhedron (mean Te–O bond length is 2.114 Å; literature value 2.251(551) Å [28]). Two Te2O₅ polyhedra share an edge (O4 and its symmetry-related counterpart) forming a centrosymmetric dimer ${}_{\propto }{}^1[$TeO_2/2O_3/1]₂ (Figure 4c). In the crystal structure, these dimers are linked to the chains involving Te1 through common corners, making up a polymeric tri-periodic oxidotellurate(IV) framework. The lone-pair electrons Ψ of both Te^IV atoms are stereochemically active and point into the free spaces of this arrangement (Figure 5). The fractional coordinates of Ψ were computed with LPLoc [33] and amount to x = −0.0038, y = 0.6334, z = 0.13323 for Ψ1 at the Te1 atom (distance Te1–Ψ1 = 0.865 Å; LP radius 1.05 Å); x = 0.5033, y = 0.8586, z = 0.1934 for Ψ2 at the Te2 atom (distance Te2–Ψ2 = 0.742 Å; LP radius 0.98 Å).

The crystal structure of β-CuTe₂O₅ has a completely different arrangement compared to that of α-CuTe₂O₅. In the latter (space group P2₁/c, Z = 4) [21], the Cu^II atom likewise shows Jahn–Teller distortions but with one shorter (2.30 Å) and one longer (2.78 Å) bond to the remote O atoms (Figure 6a). The CuO₆ octahedra share an edge to form ${}_{\propto }{}^1[$CuO_2/2O_4/1]₂ double octahedra, with the dimers arranged into rows parallel [001]. The two Te^IV atoms, again under the consideration of only tightly bound O atoms, each have a trigonal-pyramidal environment. Through corner-sharing, a bent Te₂O₅ ditellurite group is formed with a Te–O–Te angle of 120.8°. The terminal Te–O₂ bonds are somewhat shorter than those of the Te–O–Te bridge, in agreement with many examples in the literature [1]. One additional O atom is in the outer coordination sphere of Te2 at 2.40 Å and connects the isolated ditellurite groups into [100] chains (Figure 6b).

The Cu₂O₁₀ dimers and the ditellurite units are linked through corner- and edge-sharing into a tri-periodic framework, again with the lone-pair electrons Ψ of both Te atoms stereochemically active (coordinates calculated with LPLoc [33]: x = 0.0920, y = 0.1104, z = 0.1943 for Ψ1 at the Te1 atom (distance Te1–Ψ1 = 1.257 Å; LP radius 1.26 Å); x = 0.7149, y = 0.3671, z = 0.2078 for Ψ2 at the Te2 atom (distance Te2–Ψ2 = 1.16 Å; LP radius 1.18 Å). The crystal structure of α-CuTe₂O₅ is shown in Figure 7.

The different structural arrangements of the building blocks in the two polymorphs make it appear very unlikely that a possible structural phase transition is enantiotropic. Based on the empirically derived Ostwald–Volmer rule, according to which the polymorph with the lower density is metastable, it might be assumed that β-CuTe₂O₅ (density 5.91 g/cm⁻³ (V/Z = 112.1 ų) versus 5.76 g/cm⁻³ (V/Z = 115.04 ų) for α-CuTe₂O₅) represents the thermodynamically stable form. There are, however, many known exceptions to this rule, e.g., graphite has a much lower density of 2.3 g cm^–3 compared to diamond with 3.5 g cm^–3, but it is the thermodynamically stable form. Moreover, the fact that CuTe₂O₅ has, so far, only been described in the form of the α-modification [21] and as naturally occurring rajite [22] speaks against the assumption that β-CuTe₂O₅ is the thermodynamically stable modification. It should be mentioned that in hydrothermal experiments for the production of α-CuTe₂O₅ (rajite) using CuSO₄ and TeO₂ as starting materials, “another phase or phases whose diffraction peaks do not match any records in the database” were also obtained in some of the experiments [34]. It does not seem unlikely that this unknown phase could be β-CuTe₂O₅.

3.6. Raman Spectroscopy of α- and β-CuTe₂O₅

The Raman spectra of α- and β-CuTe₂O₅ are shown in Figure 8, and the proposed assignments of Raman bands are compiled in

Table 4. Assignment of Raman bands.

α-CuTe2O5		β-CuTe2O5
Band position/cm−1	Assignment	Band position/cm−1	Assignment
763 sh, 744 vs	νs(TeO2)	767 sh, 719 vs	ν(Te–O)
680 m, 665 m	νas(TeO2)	656 w	νas(Te–O–Te)
595 vw	νas(Te–O–Te)	604 s	νs(Te–O–Te)
507 vvw	νs(Te–O–Te)	475 m, 431 m, 397 m	ν(Te2O2) + δ(TeOn)
441 vs	δ(TeO2)	314 vs	δ(TeOn)
375 s, 348 sh, 311 vw, 292 w	“Rocking”	265 sh, 245 m, 211 s, 175 w, 145 s, 111 s	δ(bridges) + τ + lattice vibrations
233 m, 216 m, 182 m	τ + δ(TeOTe) + lattice vibrations

vs: very strong; s: strong, m: medium; w: weak; vw: very weak; vvw: very very weak; sh: shoulder.

.

The general spectral pattern of α-CuTe₂O₅ is similar to that previously reported for MgTe₂O₅ [35], but interestingly, band positions are clearly shifted to lower wave numbers for α-CuTe₂O₅. This effect can probably be related to the differences in the outer oxygen coordination of the Te^IV atoms in both structures; also, all Te–O bond lengths are slightly longer for the two Te^IV atoms in α-CuTe₂O₅ (1.877, 1.883, 1.931; 1.859, 1.866, 2.019, 2.402 Å [21]) than for the unique Te^IV atom in MgTe₂O₅ (1.8498 (11), 1.8588 (12), 1.9900 (9), 2.4048 (13) Å [36]).

In the crystal structure of α-CuTe₂O₅, the shape of the ditellurite anion is analogous to that of diselenite, Se₂O₇^2– (cf. for example [35,37]). Fifteen internal vibrations are expected for the seven-atomic Te₂O₅^2– anion as follows: two antisymmetric vibrations of the terminal TeO₂ groups, two symmetrical vibrations of the terminal TeO₂ groups, one antisymmetric Te–O–Te bridge vibration, one symmetric Te–O–Te bridge vibration, two deformations of the terminal TeO₂ groups, one deformation of the Te–O–Te bridge, four O–TeO₂ “rocking” vibrations, and two torsional vibrations. As the real symmetry of the anion in the crystal lattice is C₁, the appearance of all these vibrations in the Raman effect can be expected. Notwithstanding and as usually observed in the case of crystal spectra, a reduction or an increase in the number of bands is also possible due to factor group effects and/or intensity problems.

As already discussed [36], it is assumed that for the terminal TeO₂-groups ν_s > ν_as in a similar way as for the analogous Se₂O₅²⁻ anion [37]. On the other hand, this assumption is clearly supported by intensity criteria since symmetric stretching modes in the Raman effect are usually very intense. For this reason, the most intense Raman band at 744 cm^–1 is assigned to this mode. The weak shoulder on the higher energy side of this band (763 cm^–1) is probably due to correlation field effects. The next doublet at 680 and 665 cm⁻¹ is assigned to components of the corresponding antisymmetric stretching.

Regarding the Te–O–Te bridge motions, they are usually very weak in the Raman spectra [35], a fact also observed in the case of diselenites [37]. We have tentatively assigned the weak band at 595 cm^–1 to the antisymmetric mode, whereas the corresponding symmetric mode is only seen as a very weak feature at 507 cm^–1. The relative positions of these two vibrations are also supported by a previous estimation based on a simple molecular model, showing the dependence of these two vibrations from the Te–O–Te angle [35]. The deformation mode of this bridge cannot be identified with certainty, but it probably lies near or below 200 cm^–1 [35].

In the lower energy region, the strongest Raman band (441 cm^–1) is assigned to one of the expected TeO₂ deformational modes, whereas the four expected “rocking” motions have been identified between 375 and 292 cm^–1. The remaining band includes the expected torsional modes, the Te–O–Te bridge vibration, and some external (“lattice”) modes.

It is interesting to mention that the present Raman spectrum of synthetic α-CuTe₂O₅ closely resembles one of the spectra recorded for a sample of the mineral rajite. However, the corresponding analysis of these spectra is rather incomplete as the authors have not made structural considerations and based their assignment only on comparisons with the TeO₃^2– anion, completely neglecting the presence of bands related to the Te₂O₅^2– bridge vibrations [38].

The higher structural complexity of β-CuTe₂O₅ is clearly reflected in its Raman spectrum. For the sake of simplicity, only the Te–O bonds from the inner coordination sphere are considered relevant for the following analysis. In this case, the polymeric anion is constructed by the condensation of trigonal-pyramidal Te1O₃ units with (Te2)₂O₄ units formed by edge-sharing of two trigonal-pyramidal TeO₃ moieties.

The strongest Raman line (719 cm^–1) can undoubtedly be related to the terminal Te–O bonds. Also, in this case, the weak shoulder preceding this band at higher energy may be generated by correlation field effects. The second group of bands can be assigned to Te–O–Te bridges involving the chains of condensed Te(1)O₃ moieties. They appear better defined and at somewhat higher energies than in the case of α-CuTe₂O₅, probably as a consequence of the condensation of these groups. The following three bands are of more complex origin, involving different types of vibrations. In this region, one expects vibrations of the dimeric Te(2)₂O₂ units and deformational modes of the different polyhedra. The dimers with the double bridge can ideally be treated as a tetra-atomic system with D_2h symmetry and, thus, possess six normal modes of vibration. Among them, three are Raman active: 2A_g + B_1g. A comparison with other similar systems suggests that also here the strongest Raman line, and that at 245 cm^–1, may be mixed to some extent with one of these vibrational modes [39]. Nevertheless, the vibrational modes related to these double bridges are mainly expected to lie around 450–400 cm^–1. On the other hand, in this region and, also, at some lower energy, deformational modes of the TeO_n polyhedra are expected. For comparison, δ(TeO₃) modes of “free” TeO₃²⁻ units are located at around 360–320 cm^–1 [40]. The last group of bands (300–100 cm^–1) includes torsional and deformational modes of different Te–O–Te bridges as well as external (“lattice”) vibrations.

3.7. Polymorphism Observed in Oxidotellurates

The observed polymorphism of NiTeO₄ and CuTe₂O₅ is not a new phenomenon in the oxidotellurate family. A database search for polymorphic forms of purely inorganic oxidotellurates of general formula A_xTe_yO_z (A is any other element, Te = Te^IV, Te^VI) in the Inorganic Crystal Structure Database (ICSD [41], data release 2024.1) revealed a large number of cases, some even with up to five different polymorphs (like for CaTeO₃ or SrTeO₃), as the compilation of alphabetically arranged phases and their crystal data in Table 5 shows.

Table 5. Structure data of polymorphic oxidotellurate phases with general composition A_xTe_yO_z. Cases in which the polymorph was named with Greek letters or information on the measurement temperatures was provided are marked accordingly (HT = high temperature; LT = low temperature; HP = high pressure); cases where the existence of a polymorph was described without structure data are also included.

Phase	Space Group	a/Å	b/Å	c/Å	α/°	β/°	γ/°	V/ų	Z	Ref.
Ag2Te2O6	P21/m	5.4562(5)	7.4009(7)	6.9122(7)	90	101.237(2)	90	273.77	2	[42]
	P21/n	5.9099(5)	11.6831(8)	8.0305(7)	90	100.424(7)	90	545.32	4	[42]
	P21/c	6.4255(10)	6.9852(11)	13.204(2)	90	90.090(3)	90	592.64	4	[43]
BaTeO3	P21/m	4.633(4)	5.943(5)	7.104(5)	90	106.4(1)	90	187.64	2	[44]
	Pnma	14.784(2)	6.129(1)	12.350(2)	90	90	90	1119.05	12	[44]
BaTe2O6	Cmcm	5.569(2)	12.796(4)	7.320(3)	90	90	90	521.63	4	[45]
HP	P21/m	5.40473	7.18472	6.54906	90	115.2374	90	230.03	2	[46]
CaTeO3 α	P43	12.1070(10)	12.1070(10)	11.0911(18)	90	90	90	1625.73	20	[47]
β	P1	25.6220(4)	10.2426(2)	11.3327(2)	107.218(10)	110.245(2)	33.0190(10)	1520.42	18	[47]
β‘ 250 K	P$\overline{1}$	25.6149(4)	10.3921(2)	11.2440(2)	108.6080(10)	112.394(2)	32.3130(10)	1479.25	18	[47]
γ	P21	8.4010(17)	5.6913(11)	22.680(5)	90	110.82(3)	90	1013.58	12	[47]
δ 673 K	P21ca	13.3647(6)	6.5330(3)	8.1896(3)	90	90	90	715.04	8	[48]
CaTe2O5	P21/c	9.382(2)	5.7095(14)	11.132(3)	90	115.109(4)	90	539.95	4	[49]
mica-like										[50]
Ca4Te5O14	Pbca	10.9536(16)	16.556(2)	15.779(2)	90	90	90	2861.49	8	[51]
HP	P21/c	11.0272(1)	12.0588(1)	10.1038(1)	90	91.8490(10)	90	1342.85	4	[52]
CdTeO3 α	P21/c	7.790(1)	11.253(2)	7.418(1)	90	113.5(1)	90	596.34	8	[53]
β	Pnma	7.45850(3)	14.52185(6)	11.04584(5)	90	90	90	1196.37	16	[54]
CdTe2O5 100 K	P21/c	9.4535(5)	5.5806(3)	10.8607(5)	90	114.4300(10)	90	521.66	4	[55]
mica-like										[50]
Cd3TeO6	P21/n	5.4986(3)	5.6383(3)	8.0191(5)	90	90.00(5)	90	248.61	2	[56]
	R3	9.1620(2)	9.1620(2)	11.0736(3)	90	90	120	805.00	6	[57]
Co3TeO6	C2/c	14.8167(18)	8.8509(11)	10.3631(14)	90	94.900(10)	90	1354.06		[58]
HP	R3	5.1937(6)	5.1937(6)	13.8237(17)	90	90	120	322.93	3	[59]
Co5TeO8	Fd3m	8.55719(19)	8.55719(19)	8.55719(19)	90	90	90	626.61	12	[60]
	P4332	8.55185(4)	8.55185(4)	8.55185(4)	90	90	90	625.42	4	[60]
Cs2TeO4	Pcmn	11.6982(15)	6.6747(9)	8.5018(9)	90	90	90	663.83	4	[61]
773 K	P63/mmc	6.8661(8)	6.8661(8)	8.897(1)	90	90	120	363.24	2	[61]
CuTeO3	Pmcn	7.604(6)	5.837(4)	12.705(6)	90	90	90	563.91	8	[62]
	P21/n	5.214(1)	9.108(2)	5.965(1)	90	95.06(1)	90	282.17	4	[63]
CuTe2O5 α	P21/c	6.871(2)	9.322(2)	7.602(2)	90	109.08(1)	90	460.17	4	[21]
β	P21/c	6.6599(4)	7.6233(4)	8.8413(5)		92.6910(10)		448.38(4)	4	this work
Cu3TeO6	Ia$\overline{3}$	9.537(1)	9.537(1)	9.537(1)	90	90	90	867.43	8	[64]
	Pcca	9.745(3)	9.749(2)	9.771(2)	90	90	90	928.28	8	[65]
HP	Ibca	9.2444(3)	9.3018(1)	9.3757(4)	90	90	90	806.21	8	[66]
Hg2Te2O7 α	C2/c	12.910(4)	7.407(2)	13.256(4)	90	112.044(5)	90	1174.93	8	[67]
β	Aba2	7.4405(12)	23.713(4)	13.522(2)	90	90	90	2385.78	16	[67]
Li2Te2O5	P21/n	10.355(3)	4.702(1)	10.860(3)	90	110.13(1)	90	496.46	4	[68]
	Pnaa	5.194(1)	8.170(2)	24.165(5)	90	90	90	1025.44	8	[68]
Mg3TeO6	R3	8.615(3)	8.615(3)	10.315(3)	90	90	120	663.00	6	[69]
HP	R3	5.1382(7)	5.1382(7)	13.8070(19)	90	90	120	315.68	3	[70]
MnTeO3 α	Pbca	10.0662(3)	8.0958(2)	14.1654(4)	90	90	90	1154.39	16	[71]
β HP	Pnma	6.1443(4)	7.7866(7)	5.4408(4)	90	90	90	260.30	4	[72]
γ	Pbca	7.4535(12)	6.4164(11)	12.843(2)	90	90	90	614.21	8	[71]
MnTe2O5 α	P42/nbc	8.82(5)	8.82(5)	13.04(5)	90	90	90	1014.41	8	[73]
β	Pbcn	7.3114(4)	10.9216(6)	6.1711(3)	90	90	90	492.78	4	[74]
Mn2TeO6 HT	P42/mnm	4.64205(11)	4.64205(11)	9.0750(2)	90	90	90	195.54	2	[75]
LT	P21/c	9.102948(10)	13.046326(9)	6.465927(6)	90	90.0331(4)	90	767.88	8	[75]
Mn3TeO6	R$\overline{3}$	8.8673(10)	8.8673(10)	10.6729(12)	90	90	120	726.77	6	[76]
HP	P21/n	5.29370(1)	5.45203(1)	7.80894(1)	90	89.62514(5)	90	225.37	2	[77]
MoTe2O7 α	P21/c	4.286	8.618	15.945	90	95.68	90	586.06	4	[78]
	Pna21	17.5632(5)	4.9107(2)	12.9518(4)	90	90	90	1117.06	8	[79]
Mo5TeO16	P21/c	10.038(2)	14.431(2)	8.1617(6)	90	90.85(1)	90	1182.16	4	[80]
	Pm2a	20.010(1)	4.0650(2)	7.2254(4)	90	90	90	587.72	2	[81]
Na2TeO4 α	P21/c	10.632(5)	5.161(2)	13.837(11)	90	103.27(4)	90	738.99	8	[82]
β	Pbcn	5.798(6)	12.24(1)	5.214(5)	90	90	90	370.02	4	[83]
γ	Pnna	13.5390(7)	8.4169(4)	6.7781(4)	90	90	90	772.40	8	[84]
NiTeO4 α	P21/c	5.5285(6)	4.6500(5)	5.5449(6)	90	113.120(2)	90	131.10(2)	2	this work
β	I41/a	9.2627(5)	9.2627(5)	6.0988(4)	90	90	90	523.26(7)	8	this work
Ni3TeO6	R3	5.103(2)	5.103(2)	13.755(10)	90	90	120	310.20	3	[18]
HP	Pnma	5.9588(1)	7.5028(1)	5.2143(1)	90	90	90	233.12	4	[85]
PbTeO3 α	C2/c	26.73	4.6	18.06	90	106	90	2134.60	24	[86]
β	P41	5.304(3)	5.304(3)	11.900(6)	90	90	90	334.78	4	[87]
γ	P$\overline{1}$	7.0185(3)	10.6166(4)	11.9616(5)	78.548(3)	82.992(2)	84.048(2)	864.12	10	[88]
Sc2TeO6	P321	8.7406(7)	8.7406(7)	4.7985(4)	90	90	120	317.48	3	[89]
HP		7.2943(3)	5.1252(2)	10.9502(4)	90	103.8800(10)	90	397.41	4	[90]
SrTeO3 α	C2	28.151(6)	5.8970(10)	15.261(3)	90	122.09(3)	90	2146.35	24	[91]
β 473 K	C2/c	28.206(6)	5.9210(10)	28.528(6)	90	114.16(3)	90	4347.06	48	[92]
γ 583 K	C2	28.262(6)	5.9350(10)	15.434(3)	90	122.21(3)	90	2190.40	24	[93]
δ 780 K	C2/m	28.438(6)	5.9500(10)	15.550(3)	90	122.45(3)	90	2220.32	24	[94]
ε	P21/c	6.7759(1)	7.2188(1)	8.6773(2)	90	126.4980(7)	90	341.20	4	[95]
Te(OH)6	P21/n	6.495(1)	9.320(1)	11.393(1)	90	133.88	90	497.10	4	[96]
	F4132	15.699(2)	15.699(2)	15.699(2)	90	90	90	3869.15	32	[97]
Te3O3(PO4)2	P21/c	12.375(2)	7.317(1)	9.834(1)	90	98.04(1)	90	881.70	4	[98]
β 203 K	P21/n	11.115(5)	4.7033(19)	17.287(7)	90	106.086(5)	90	868.33	4	[99]
Tl2Te2O5 α	P21/n	7.119(1)	12.138(2)	8.439(2)	90	114.28(3)	90	664.72	4	[100]
β										[101]

Table 5. Structure data of polymorphic oxidotellurate phases with general composition A_xTe_yO_z. Cases in which the polymorph was named with Greek letters or information on the measurement temperatures was provided are marked accordingly (HT = high temperature; LT = low temperature; HP = high pressure); cases where the existence of a polymorph was described without structure data are also included.

Phase	Space Group	a/Å	b/Å	c/Å	α/°	β/°	γ/°	V/ų	Z	Ref.
Ag2Te2O6	P21/m	5.4562(5)	7.4009(7)	6.9122(7)	90	101.237(2)	90	273.77	2	[42]
	P21/n	5.9099(5)	11.6831(8)	8.0305(7)	90	100.424(7)	90	545.32	4	[42]
	P21/c	6.4255(10)	6.9852(11)	13.204(2)	90	90.090(3)	90	592.64	4	[43]
BaTeO3	P21/m	4.633(4)	5.943(5)	7.104(5)	90	106.4(1)	90	187.64	2	[44]
	Pnma	14.784(2)	6.129(1)	12.350(2)	90	90	90	1119.05	12	[44]
BaTe2O6	Cmcm	5.569(2)	12.796(4)	7.320(3)	90	90	90	521.63	4	[45]
HP	P21/m	5.40473	7.18472	6.54906	90	115.2374	90	230.03	2	[46]
CaTeO3 α	P43	12.1070(10)	12.1070(10)	11.0911(18)	90	90	90	1625.73	20	[47]
β	P1	25.6220(4)	10.2426(2)	11.3327(2)	107.218(10)	110.245(2)	33.0190(10)	1520.42	18	[47]
β‘ 250 K	P$\overline{1}$	25.6149(4)	10.3921(2)	11.2440(2)	108.6080(10)	112.394(2)	32.3130(10)	1479.25	18	[47]
γ	P21	8.4010(17)	5.6913(11)	22.680(5)	90	110.82(3)	90	1013.58	12	[47]
δ 673 K	P21ca	13.3647(6)	6.5330(3)	8.1896(3)	90	90	90	715.04	8	[48]
CaTe2O5	P21/c	9.382(2)	5.7095(14)	11.132(3)	90	115.109(4)	90	539.95	4	[49]
mica-like										[50]
Ca4Te5O14	Pbca	10.9536(16)	16.556(2)	15.779(2)	90	90	90	2861.49	8	[51]
HP	P21/c	11.0272(1)	12.0588(1)	10.1038(1)	90	91.8490(10)	90	1342.85	4	[52]
CdTeO3 α	P21/c	7.790(1)	11.253(2)	7.418(1)	90	113.5(1)	90	596.34	8	[53]
β	Pnma	7.45850(3)	14.52185(6)	11.04584(5)	90	90	90	1196.37	16	[54]
CdTe2O5 100 K	P21/c	9.4535(5)	5.5806(3)	10.8607(5)	90	114.4300(10)	90	521.66	4	[55]
mica-like										[50]
Cd3TeO6	P21/n	5.4986(3)	5.6383(3)	8.0191(5)	90	90.00(5)	90	248.61	2	[56]
	R3	9.1620(2)	9.1620(2)	11.0736(3)	90	90	120	805.00	6	[57]
Co3TeO6	C2/c	14.8167(18)	8.8509(11)	10.3631(14)	90	94.900(10)	90	1354.06		[58]
HP	R3	5.1937(6)	5.1937(6)	13.8237(17)	90	90	120	322.93	3	[59]
Co5TeO8	Fd3m	8.55719(19)	8.55719(19)	8.55719(19)	90	90	90	626.61	12	[60]
	P4332	8.55185(4)	8.55185(4)	8.55185(4)	90	90	90	625.42	4	[60]
Cs2TeO4	Pcmn	11.6982(15)	6.6747(9)	8.5018(9)	90	90	90	663.83	4	[61]
773 K	P63/mmc	6.8661(8)	6.8661(8)	8.897(1)	90	90	120	363.24	2	[61]
CuTeO3	Pmcn	7.604(6)	5.837(4)	12.705(6)	90	90	90	563.91	8	[62]
	P21/n	5.214(1)	9.108(2)	5.965(1)	90	95.06(1)	90	282.17	4	[63]
CuTe2O5 α	P21/c	6.871(2)	9.322(2)	7.602(2)	90	109.08(1)	90	460.17	4	[21]
β	P21/c	6.6599(4)	7.6233(4)	8.8413(5)		92.6910(10)		448.38(4)	4	this work
Cu3TeO6	Ia$\overline{3}$	9.537(1)	9.537(1)	9.537(1)	90	90	90	867.43	8	[64]
	Pcca	9.745(3)	9.749(2)	9.771(2)	90	90	90	928.28	8	[65]
HP	Ibca	9.2444(3)	9.3018(1)	9.3757(4)	90	90	90	806.21	8	[66]
Hg2Te2O7 α	C2/c	12.910(4)	7.407(2)	13.256(4)	90	112.044(5)	90	1174.93	8	[67]
β	Aba2	7.4405(12)	23.713(4)	13.522(2)	90	90	90	2385.78	16	[67]
Li2Te2O5	P21/n	10.355(3)	4.702(1)	10.860(3)	90	110.13(1)	90	496.46	4	[68]
	Pnaa	5.194(1)	8.170(2)	24.165(5)	90	90	90	1025.44	8	[68]
Mg3TeO6	R3	8.615(3)	8.615(3)	10.315(3)	90	90	120	663.00	6	[69]
HP	R3	5.1382(7)	5.1382(7)	13.8070(19)	90	90	120	315.68	3	[70]
MnTeO3 α	Pbca	10.0662(3)	8.0958(2)	14.1654(4)	90	90	90	1154.39	16	[71]
β HP	Pnma	6.1443(4)	7.7866(7)	5.4408(4)	90	90	90	260.30	4	[72]
γ	Pbca	7.4535(12)	6.4164(11)	12.843(2)	90	90	90	614.21	8	[71]
MnTe2O5 α	P42/nbc	8.82(5)	8.82(5)	13.04(5)	90	90	90	1014.41	8	[73]
β	Pbcn	7.3114(4)	10.9216(6)	6.1711(3)	90	90	90	492.78	4	[74]
Mn2TeO6 HT	P42/mnm	4.64205(11)	4.64205(11)	9.0750(2)	90	90	90	195.54	2	[75]
LT	P21/c	9.102948(10)	13.046326(9)	6.465927(6)	90	90.0331(4)	90	767.88	8	[75]
Mn3TeO6	R$\overline{3}$	8.8673(10)	8.8673(10)	10.6729(12)	90	90	120	726.77	6	[76]
HP	P21/n	5.29370(1)	5.45203(1)	7.80894(1)	90	89.62514(5)	90	225.37	2	[77]
MoTe2O7 α	P21/c	4.286	8.618	15.945	90	95.68	90	586.06	4	[78]
	Pna21	17.5632(5)	4.9107(2)	12.9518(4)	90	90	90	1117.06	8	[79]
Mo5TeO16	P21/c	10.038(2)	14.431(2)	8.1617(6)	90	90.85(1)	90	1182.16	4	[80]
	Pm2a	20.010(1)	4.0650(2)	7.2254(4)	90	90	90	587.72	2	[81]
Na2TeO4 α	P21/c	10.632(5)	5.161(2)	13.837(11)	90	103.27(4)	90	738.99	8	[82]
β	Pbcn	5.798(6)	12.24(1)	5.214(5)	90	90	90	370.02	4	[83]
γ	Pnna	13.5390(7)	8.4169(4)	6.7781(4)	90	90	90	772.40	8	[84]
NiTeO4 α	P21/c	5.5285(6)	4.6500(5)	5.5449(6)	90	113.120(2)	90	131.10(2)	2	this work
β	I41/a	9.2627(5)	9.2627(5)	6.0988(4)	90	90	90	523.26(7)	8	this work
Ni3TeO6	R3	5.103(2)	5.103(2)	13.755(10)	90	90	120	310.20	3	[18]
HP	Pnma	5.9588(1)	7.5028(1)	5.2143(1)	90	90	90	233.12	4	[85]
PbTeO3 α	C2/c	26.73	4.6	18.06	90	106	90	2134.60	24	[86]
β	P41	5.304(3)	5.304(3)	11.900(6)	90	90	90	334.78	4	[87]
γ	P$\overline{1}$	7.0185(3)	10.6166(4)	11.9616(5)	78.548(3)	82.992(2)	84.048(2)	864.12	10	[88]
Sc2TeO6	P321	8.7406(7)	8.7406(7)	4.7985(4)	90	90	120	317.48	3	[89]
HP		7.2943(3)	5.1252(2)	10.9502(4)	90	103.8800(10)	90	397.41	4	[90]
SrTeO3 α	C2	28.151(6)	5.8970(10)	15.261(3)	90	122.09(3)	90	2146.35	24	[91]
β 473 K	C2/c	28.206(6)	5.9210(10)	28.528(6)	90	114.16(3)	90	4347.06	48	[92]
γ 583 K	C2	28.262(6)	5.9350(10)	15.434(3)	90	122.21(3)	90	2190.40	24	[93]
δ 780 K	C2/m	28.438(6)	5.9500(10)	15.550(3)	90	122.45(3)	90	2220.32	24	[94]
ε	P21/c	6.7759(1)	7.2188(1)	8.6773(2)	90	126.4980(7)	90	341.20	4	[95]
Te(OH)6	P21/n	6.495(1)	9.320(1)	11.393(1)	90	133.88	90	497.10	4	[96]
	F4132	15.699(2)	15.699(2)	15.699(2)	90	90	90	3869.15	32	[97]
Te3O3(PO4)2	P21/c	12.375(2)	7.317(1)	9.834(1)	90	98.04(1)	90	881.70	4	[98]
β 203 K	P21/n	11.115(5)	4.7033(19)	17.287(7)	90	106.086(5)	90	868.33	4	[99]
Tl2Te2O5 α	P21/n	7.119(1)	12.138(2)	8.439(2)	90	114.28(3)	90	664.72	4	[100]
β										[101]

The causes and structural effects of polymorphism in the listed oxidotellurates are manifold—preparation conditions, temperature and/or pressure-dependent thermodynamic (meta-)stabilities, monotropic/enantiotropic transitions, reconstructive/displacive transitions, just to name but a few—and depend on an individual case. A detailed consideration is beyond the scope of this brief overview, and the interested reader is, therefore, referred to the original literature.

4. Conclusions

The original aim of this study, namely, the growth of NiTeO₄ and CuTeO₄ crystals as single-phase material, could not be achieved. At the selected average temperature of 620 °C under CVTR conditions, NiTeO₄ is at the limit of its thermal stability, whereas CuTeO₄ has no stability range under the selected hydrothermal conditions at 210 °C. This explains the observation that instead of the expected phases NiTeO₄ and CuTeO₄, single crystals of NiTe₂O₅ and of NiTeO₄ (in the form of the known α-polymorph and of a previously unknown β-polymorph) and of so far unknown β-CuTe₂O₅, respectively, were obtained in very small amounts. Apart from crystal structure determinations from single-crystal X-ray data for α-, β-NiTeO₄, and β-CuTe₂O₅, as well as Raman measurements for the latter, the scarcity of available material made further investigations and measurements of physical properties impossible. Hence, experimental details for the growth of α-, β-NiTeO₄, and β-CuTe₂O₅ single crystals in larger quantities and as single-phase material must be optimized in the future. These time-consuming experiments could not be carried out as part of the present structural study. Only through the successful synthesis of the individual polymorphic forms will it be possible to obtain precise information about their thermodynamic stabilities and their thermal behaviors in relation to any occurring phase transitions.