Mineral-like Synthetic Compounds Stabilized under Hydrothermal Conditions: X-ray Diffraction Study and Comparative Crystal Chemistry

Abstract

Under hydrothermal conditions emulating natural hydrothermalites, three oxo-salts with sodium and transition metal cations were obtained in the form of single crystals. Their compositions and crystal structures were studied using scanning electron microscopy, microprobe X-ray spectral analysis, and X-ray single-crystal diffraction. The sodium cobalt silicate, i.e., Na₂CoSiO₄, a structural analog of the mineral liberite, is well known as an ionic conductor. Its crystal structure consists of a framework derived from β-tridymite, built using the Co- and Si-centered tetrahedra sharing vertices. The sodium oxocuprate phosphate chloride Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl belongs to a group of compounds, including fumarolic minerals, characterized by the presence of oxo-centered pyroxene-like chains in their structures. The crystal structure of mineralogically probable sodium vanadium phosphate hydroxide (Na₃V(OH)(HPO₄)(PO₄)) is based on chains built using octahedra centered by magnetically active V³⁺. Magnetic susceptibility measurements indicate an antiferromagnetic arrangement of V³⁺ ions and no transition to an ordered state up to 2 K.

1. Introduction

Nature has long served as a source of inspiration for scientists. Minerals, renowned for their geological stability, have played a significant role in the chemistry of materials. However, it is a well-known fact, both among materials scientists and mineralogists, that natural compounds often contain a multitude of impurities. These impurities frequently interfere with the study of the physical properties of minerals, hindering their industrial applications. To solve this problem, the laboratory gradually adopted an experimental approach that mimics the natural conditions of mineral formation.

The natural pegmatite and hydrothermal process occur in superheated water under pressure beneath the earth’s surface in a temperature range of 100–700 °C. The hydrothermal technology takes its name from hydrothermal vents where extreme temperatures and pressures occur. A distinctive feature of hydrothermal vents is the presence of supercritical water (374 °C and a pressure of 218 atm), with its unique properties to dissolve solids like a liquid yet flow with next to no viscosity like a gas. By working at the upper end of the temperature range (around 500–700 °C), it is possible to achieve conditions roughly similar to those found in natural pegmatite systems. Even under these temperature (and pressure) conditions, the liquid remains sufficiently polar to partially dissolve most metal oxyanions. Synthesis at intermediate temperatures of 100 °C < T < 374 °C allows it to remain well below the critical point of water but still heat the vessel above the normal boiling point. By keeping the reaction closed, water can be maintained in a liquid state well above the point at which it would normally boil. As long as the critical point is avoided, the pressures required for this approach are much more controllable. Undoubtedly, both thermal regimes are characteristic of the geological system [1].

These temperatures and chemical environments inherent in the hydrothermal process can be simulated in the laboratory. Therefore, by exploiting the similarities between mineral forming conditions and laboratory techniques, it becomes possible to investigate the structural relationships with complex structures derived from hydrothermal fluids, using the natural materials as a motivation. This approach allows for the creation of chemically “pure” analogs of minerals from various chemical classes, including rock-forming oxides and silicates, as well as their recrystallized products. Thus, silicates of rare-earth metals, structurally related to minerals such as wadeite, fresnoite, apatite, olivine, and garnet, were obtained using high-temperature hydrothermal techniques. The synthetic analogues have found applications as phosphors, lasers, scintillators, and Faraday rotators due to their optical properties [2]. In our research into crystallization processes at medium temperatures, we successfully produced structural analogs of minerals such as mahnertite (NaCu₃(AsO₄)₂Cl·5H₂O) [3] and ellenbergerite (Mg₆(Mg,Ti,Zr)₂(Al,Mg)₆Si₈O₂₈(OH)₁₀) [4]. These synthetic phases exhibit unique magnetic properties, particularly low-dimensional magnetism arising from the presence of chains or layers of octahedra centered by open-shell metal atoms in their structures. Past reviews of synthetic mineral analogs [5,6] highlighted a broad range of compounds with ion-conducting properties. The synthetic equivalents of minerals such as triphylite (LiFePO₄), niahite ((NH₄)(Mn,Mg)(PO₄)·H₂O), simferite (LiMg(PO₄)), tavorite (LiFe(PO₄)(OH)), marićite (NaFe(PO₄)), sarcopside ((Fe,Mn,Mg)₃(PO₄)₂), and alluaudite ((Na,Ca)Mn(Fe,Mn,Fe,Mg)₂(PO₄)₃) are actively utilized as electrode materials for sodium- or lithium-ion batteries.

Evidently, there is a substantial interest in the field of materials science not only in direct mineral analogs but also in determining their structural derivatives. By modifying the natural conditions of mineral formation in laboratory experiments by adjusting factors such as chemical reagents, temperature, pressure, and mineralizers, a wide array of mineralogically plausible compounds can be obtained. In this paper, we present the experimental results of crystallization in hydrothermal systems that mimic the expected hydrothermal conditions. We successfully synthesized three sodium-containing compounds: Na₂CoSiO₄ (I), a structural analog of mineral liberite (Li₂BeSiO₄) and the mineralogically reasonable phosphates Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl (II) and Na₃V(OH)(HPO₄)(PO₄) (III). All investigated phases were previously obtained in the form of polycrystalline samples, and their crystal structures were studied using powder X-ray diffraction. The synthesis of these compounds in the form of single crystals allowed us to obtain more precise structural data, localize hydrogen atoms, refine their positions, and analyze hydrogen bonds. Moreover, studies have been carried out on the magnetic properties of V³⁺-containing phosphate.

2. Materials and Methods

2.1. Hydrothermal Synthesis

Single crystals were obtained in three different hydrothermal phosphate and silicate systems with cations of alkali and transition metals (

Table 1. Secondary-electron SEM images showing the sample morphologies and experimental conditions of hydrothermal synthesis.

(I)	(II)	(III)
Na2CoSiO4	Na2(Cu+0.8Na0.2)Cl[Cu2+3O(PO4)2]	Na3V(OH)(HPO4)(PO4)
T 450 °C, P 500 atm	T 450 °C, P 500 atm	T 270 °C, P 70 atm
2 g CoCl2 (15.4 mmol) 1 g SiO2 (16.7 mmol) NaOH solution	3 g CuCl2 (22.3 mmol) 1 g CuCl (10.1 mmol) 2 g (12.2 mmol) Na3PO4 HCl solution	1 g V2O3 (6.6 mmol) 2 g Na3PO4 (12 mmol) 0.2 g Li2CO3 (3 mmol) H2O
pH 12	pH 1.5	pH 3
Copper-lined chrome-nickel autoclave, V 14 mL Autoclave filling 50%	Copper-lined chrome-nickel autoclave, V 14 mL Autoclave filling 50%	Steel autoclave lined with fluoroplastic, V 7 mL Autoclave filling 72%

). The initial chemically pure reagents were weighed, thoroughly ground in an agate mortar, and placed into an autoclave. In the silicate system, sodium hydroxide was added to improve the solubility of SiO₂, raising the solution’s pH to 12. In the copper phosphate system, a small amount of hydrochloric acid solution was used to achieve an acidic medium with a pH of 1.5. In the phosphate system with vanadium, a small amount of lithium carbonate served as a mineralizer. Different temperature regimes were used for synthesizing these single crystals. For 270 °C middle-temperature hydrothermal conditions, the routine steel autoclave lined with fluoroplastic was utilized. However, 450 °C high-temperature hydrothermal experiments were conducted using a copper-coated high-pressure vessel of our own production made from a nickel–chromium alloy. It is critical for safety reasons to use superalloy autoclaves, as the creep rupture stress significantly enlarges with the increase in temperature. Distilled water was added according to the selected percentage of autoclave volume filling required to achieve the desired internal pressure. All experiments lasted for 10 days, after which period the autoclaves were left to cool naturally for 24 h. Crystallization products were washed multiple times with warm distilled water and air-dried. All three phases are shown in Table 1, where they are denoted as (I), (II), and (III), respectively.

2.2. X-ray Spectral Analysis

High-quality crystals selected via light microscopy were subjected to X-ray spectral analysis. Semi-quantitative ED analysis and crystal images were obtained from unpolished samples through graphite sputtering, using an accelerating voltage of 20 kV and a current rate of 10 nA, with a spectrum accumulation time of 50 s. These measurements were conducted at the Laboratory of Analytical Techniques of High Spatial Resolution, the Department of Petrology, the Faculty of Geology, Lomonosov Moscow State University, utilizing a JEOL JSM-6480LV scanning electron microscope with EDS-spectrometer. As reference standards, stoichiometric compounds and natural minerals were used, such as metallic Co and Cu, NaCl for Na and Cl, V₂O₃ for V, GaP for P, and natural garnet and diopside for Si and O, respectively.

2.3. Single-Crytsal X-ray Diffraction

The obtained crystals were investigated using X-ray diffraction at low temperatures for (I) and (II) and in standard conditions for (III), using MoKα radiation with a single-crystal diffractometer equipped with a CCD detector (Atlas-S2 & Sapphire3 detectors, Rigaku Oxford Diffraction, Tokyo, Japan). The datasets were corrected for background, the Lorentz and polarization effects, and absorption [7]. Most calculations for structural studies were performed using the WinGX program system [8]. The crystal structures were solved via direct methods and refined against the F² data using the SHELX programs [9,10]. Structural data were deposited via the joint CCDC/FIZ Karlsruhe deposition service under the deposition numbers 2310942 (I), 2310943 (II), and 2310944 (III). Cif-data can be obtained free of charge from FIZ Karlsruhe via the following webpage: www.ccdc.cam.ac.uk/structures, accessed on 29 November 2023. Also, cif and checkcif files can be found in the Supplementary Materials to this paper.

3. Results and Discussion

3.1. Chemical Composition

The X-ray spectral analysis of blue lamellar crystals (I) revealed the presence of Na, Co, Si, and O atoms; the brown isometric well-faced crystals (II) contained Na, Cu, P, Cl, and O in a Na:Cu:P:Cl ratio of 2.2:3.5:2:1. The gray-green prismatic crystals in intergrowths of (III) contained Na, V, P, and O atoms with a Na:V:P ratio equal to 3:1:2.

3.2. Crystal Structure Solution

Crystal data and details of data collection and refinement are presented in

Table 2. The crystal data and experimental details of X-ray structural studies.

Structural Formula	(I)	(II)	(III)
	Na2CoSiO4	Na2Cu3O(Cu0.8Na0.2)(PO4)2Cl	Na3V(OH)(HPO4)(PO4)
Mr	197.00	531.00	327.87
Space group, Z	Pn, 2	Cmcm, 4	C2/m, 4
Temperature (K)	170	150	293
Unit cell parameters a, b, c (Å)	5.2271(5)	13.6243(2)	15.4157(10)
	5.4198(4)	10.3531(2)	7.3107(4)
	7.0466(6)	6.3586(1)	7.0556(4)
β (°)	90.011(7)	90	96.702(6)
V (Å3)	199.63(3)	896.90(3)	789.73 (8)
Radiation	Mo Kα	Mo Kα	Mo Kα
µ (mm−1)	4.69	9.56	1.86
Crystal size (mm)	0.15 × 0.06 × 0.02	0.17 × 0.11 × 0.07	0.19 × 0.09 × 0.06
Diffractometer	Oxford Diffraction Gemini		Xcalibur Sapphire3
Number of reflections: measured independently based on [I > 2σ(I)]	1800, 909, 894	8798, 732, 727	1352, 1352, 1043
Rint	0.016	0.016	-
(sin θ/λ)max (Å−1)	0.703	0.703	0.594
R [F2 > 2σ(F2)], wR(F2), S	0.025, 0.064, 1.03	0.018, 0.046, 1.29	0.043, 0.109, 1.03
Number of refined parameters	74	57	91
Δρmax, Δρmin (e Å−3)	0.46, −0.45	0.74, −0.91	0.64, −0.57
Flack parameter *	0.01(3) *	-	-

* The algorithm from [11] was used to determine the Flack parameter x.

. The pseudo-orthorhombic (I) Na₂CoSiO₄ was refined as pseudo-merohedral microtwin with a ratio of twin components equal to 0.32:0.68 for R₁ = 0.025. During the solution of the crystal structure (II), three symmetrically independent positions of copper atoms were recognized. The coordination geometry for one of these positions (Cu3) indicated the presence of monovalent copper, while the other two positions (Cu1, Cu2) exhibited distorted Jahn–Teller polyhedra typical of Cu²⁺. The refinement revealed that the Cu3⁺ site is “diluted” by sodium atoms at a ratio of 0.8:0.2. The refinement resulted in the crystal–chemical formula Na₂(Cu⁺₀.₈Na₀.₂)Cl[Cu²⁺₃O(PO₄)₂] for (II). Data collection for the monoclinic phase (III) revealed non-merohedral twinning, connected by a pseudo-2-fold axis. A twin ratio between two components was refined at a ratio of 0.49:0.51. Two hydrogen atoms corresponding to two symmetrically independent OH groups were located via the electron density difference syntheses and refined based on the isotropic approximation. The final formula established through structural refinement (R₁ = 0.043) was Na₃V(OH)(HPO₄)(PO₄). The formulae refined during the X-ray diffraction structural study of the compounds were consistent with the results of the microprobe analysis.

3.3. Na₂CoSiO₄, a Structural Analog of the Mineral Liberite

3.3.1. The Structural Description and Analysis of Interatomic Distances

In the crystal structure of Na₂CoSiO₄, the cobalt and silicon tetrahedra share all vertices, forming the tetrahedral anionic framework [CoSiO₄]^2-, derived from the structure of β-tridymite, as illustrated in Figure 1a. The framework is negatively charged and balanced by sodium cations located within the channels. Within the bc plane, we can observe cellular layers consisting of six-membered rings composed of SiO₄ and CoO₄ tetrahedra alternating with each other (Figure 1b). Unlike the crystal structure of β-tridymite, these layers exhibit a uniform orientation of tetrahedra, described as UUUUUU (where ‘U’ signifies the upward orientation of the tetrahedra). These layers are interconnected along the a-axis through shared polyhedra vertices, maintaining the overall polarity of the structure.

In this monoclinic acentric crystal structure of Na₂CoSiO₄, all the atoms occupy general 2a Wyckoff positions. All Na, Co, and Si atoms are coordinated by oxygen ligands, forming tetrahedra (Figure 2). The Si–O bond lengths within the SiO₄ tetrahedron range from 1.618(8) to 1.644(8) Å, with an average value of 1.63 Å (

Table 3. Characteristic distances, Å, in the Na₂CoSiO₄ crystal structure.

Co–Tetrahedron		Si–Tetrahedron
Co–O2	1.928(6)	Si1–O4	1.618(8)
–O3	1.944(5)	–O3	1.627(4)
–O1	1.958(4)	–O1	1.631(7)
–O4	1.969(5)	–O2	1.644(8)
<Co–O>	1.95	<Si–O>	1.63
Na1–Tetrahedron		Na2–Tetrahedron
Na1–O2	2.245(10)	Na2–O2	2.325(6)
–O3	2.272(10)	–O4	2.344(12)
–O4	2.279(5)	–O1	2.364(11)
–O1	2.331(10)	–O3	2.407(10)
<Na1–O>	2.28	<Na2–O>	2.36

). The larger CoO₄ tetrahedron exhibits Co–O distances in the interval 1.928(6)–1.969(5) Å (average 1.95 Å).

Sodium atoms occupy two symmetrically independent positions, both surrounded by four O ligands to form tetrahedra with Na1–O distances varying between 2.245(10) and 2.331(10) Å (average 2.28 Å). Meanwhile, the Na2–O distances are larger, ranging from 2.325(6) to 2.407(10) Å (average 2.36 Å). The next-to-nearest oxygen atoms associated with the Na1 and Na2 cations are situated at distances of 2.984(10) Å and 2.908(10) Å, respectively. Bond valence calculations show the impact of these next-to-nearest oxygens on the total valence of the sodium cations (

Table 4. Na₂CoSiO₄. Bond valence data *.

	Na1	Na2	Co	Si	Σ
O1	0.203, 0.052	0.189	0.487	0.981	1.91
O2	0.242	0.205	0.528	0.947	1.92
O3	0.229	0.173, 0.061	0.506	0.992	1.96
O4	0.226	0.197	0.473	1.016	1.91
Σ	0.95	0.83	1.99	3.94

* The algorithm and empirical parameters from [12] were used.

).

Interestingly, the NaO₄ polyhedra form a cationic framework that exhibits topological similarity to the anionic framework built from Si- and Co-centered tetrahedra (Figure 1b and Figure 3a). The three-periodic vertex-sharing of NaO₄ tetrahedra and interconnected CoO₄/PO₄ tetrahedra leads to quasi-layering in (101) projection. The [CoSiO₄] quasi-layers alternate in the [$\overline{1}$01] direction with topologically equal quasi-layers of Na-centered polyhedra (Figure 3b). The electrochemical properties of Na₂CoSiO₄, which will be discussed below, are associated with the migration of Na⁺ ions through these quasi-layers parallel to the ($\overline{1}$01) planes (Figure 3).

3.3.2. Na₂CoSiO₄ and the Liberite Structural Family of Ionic Conductors

The studied silicate, namely Na₂CoSiO₄, is a full structural analog of the mineral liberite, namely Li₂BeSiO₄, discovered in southern China [13]. Both liberite and disodium cobalt silicate crystallize in monoclinic or orthorhombic modifications, and both structures exhibit tetrahedral frameworks. Remarkably, the monoclinic polymorph of Na₂CoSiO₄ is entirely ordered, unlike the orthorhombic polymorph. In the orthorhombic structure Co and Si share the same positions, though the number of Na positions twice increased, and each site is a half populated.

Liberite and Na₂CoSiO₄ belong to a large group of compounds with the general formula A₂MeXO₄, where A⁺ represents the alkaline cations Li or Na; Me²⁺ represents Zn, Mn, Co, Mg, and Be; and X⁴⁺ represents the semimetals Si or Ge. The crystal structures of A₂MeXO₄ phases are based on a distorted hexagonal close-packing of oxygen atoms, with half of the tetrahedral voids occupied by the A, Me, and X atoms. The rich crystal chemistry of this family results from variations in the occupancy of the tetrahedral voids and the bonding of A⁺-, Me²⁺-, and X⁴⁺-centered tetrahedra. Accordingly, the A₂MeXO₄ compounds crystallize in several polymorphic modifications, mainly in the monoclinic and orthorhombic space groups P2₁, P2₁/n, Pn, Pmnb, Pmn2₁, and Pbn2₁ [5]. The good Na⁺/Li⁺ ionic conductivity of the A₂MeXO₄ family arises from its structural peculiarities. Channels within the anionic tetrahedral framework allow for the migration of alkali metal cations. In these compounds, the Na⁺/Li⁺ deintercalation is compensated by two electrons per Me²⁺ transition metal [14]. Moreover, the presence of transition metal atoms determines the magnetic properties of these phases due to the super-exchange magnetic interactions between Me²⁺ cations through the [XO₄]⁴⁻ anionic tetrahedra [15,16].

There was no clarity in the space group and unit cell parameters of the monoclinic polymorph of Na₂CoSiO₄, since earlier structural studies were conducted using polycrystalline Na₂CoSiO₄ samples synthesized through solid-state reactions [17,18]. These difficulties were apparently connected to the Na₂CoSiO₄ pseudo-orthorhombic symmetry of actually monoclinic crystals with β close to 90°, which exhibited a pseudo-merohedral twinning. Recently, by means of high-resolution X-ray powder diffraction using a synchrotron radiation source, a group of researchers [19] demonstrated the splitting of (10$\overline{1}$) and (101) reflections. Additionally, the absence of a small-angle (010) reflection undoubtedly confirmed the true monoclinic symmetry and the real value of the b parameter of the unit cell. In the course of our single-crystal structural study, it was supposed that the aparent orthorhombic unit cell of the sample was actually monoclinic, with the monoclinic angle being close to 90°. Accordingly, the crystal structure was refined in an anisotropic approximation of thermal vibration in the space group Pn as a two-component pseudomerohedric twin with a volume ratio of 32:68 and an obliquity of 0.01°. Our structural data, obtained via a low-temperature single-crystal X-ray experiment, correlate with the findings of powder synchrotron studies, thereby providing undoubtable confirmation of the acentric space group Pn inherent to the monoclinic modification of Na₂CoSiO₄.

Numerous studies have focused on the various physical properties of Na₂CoSiO₄ crystals, which exhibit good semiconductor properties and are promising for use in high-power Na-ion solar cells [20]. Na₂CoSiO₄ has also been investigated as a positive electrode material for sodium-ion capacitors. It showed excellent electrochemical performance, with a specific capacity of 42 F/g and a high specific energy of 12.4 Wh/kg (at a power density of 782.7 W/kg), and excellent cycling performance, retaining up to 84% of its capacity after 1500 charge–discharge cycles [18]. The results of [17] recommend the potential use of Na₂CoSiO₄ as a cathode material for sodium-ion batteries. Electrochemical measurements using cells with metallic sodium anodes revealed a reversible specific capacity of 100 mAh/g at an average discharge voltage of 3.3 V vs. Na/Na⁺. The atomistic modeling of Na⁺ diffusion demonstrated a low activation barrier and the presence of three-periodic diffusion pathways within the silicate framework, signifying favorable Na⁺ intercalation kinetics.

3.4. Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl with Oxo-Centered Pyroxene-like Chains

3.4.1. Analysis of Interatomic Distances and Description of the Crystal Structure

The key structural units of Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl include an orthophosphate tetrahedron, two Cu²⁺-centered polyhedra, and a Cu⁺-centered trigonal bipyramid (Figure 4a). The Cu1²⁺O₆ polyhedron with 2/m symmetry is a rare example of a compressed Jahn–Teller octahedron. There are four Cu1–O bonds with lengths equal to 2.254(2) Å and two shorter bonds of 1.851(1) Å (

Table 5. The interatomic distances, Å, in the Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl structure.

Cu1 Octahedron		Cu2 Tetragonal Pyramid		Cu3 * Trigonal Bipyramid
Cu1–O3 × 2	1.852(1)	Cu2–O3	1.911(2)	Cu3–O2 × 2	2.032(2)
–O4 × 4	2.254(2)	–O1	1.921(2)	–Cl	2.394(5)
		–O4 × 2	2.022(2)	–Cl × 2	3.1875(1)
<Cu1–O>	2.12	–Cl1	2.5789(8)
		<Cu2–O>	1.97	<Cu3–O>	2.03
Na Octahedron			P Tetrahedron
Na–O2 × 2	2.309(2)		P–O2	1.532(2)
–O1 × 2	2.460(2)		–O1	1.544(2)
–O4 × 2 –Cl × 2	2.884(2) 3.270(2)		–O4 × 2	1.544(2)
<Na–O>	2.55		<P–O>	1.54

* Cu3-site is occupied by 74% Cu⁺ and 26% Na⁺.

). The polyhedron distortion through the compressed axial oxygen atoms is associated with the requirement to avoid closer interaction between four neighboring Cu2O₄Cl pyramids. The Cu2²⁺O₄Cl polyhedron is a distorted tetragonal pyramid, with the four Cu2–O distances at the base of the pyramid located in the range 1.911(2)–2.022(2) Å, and the Cl apex is located 2.579(1) Å from the Cu2.

The Cu⁺O₂Cl₃ polyhedron possesses mm2 symmetry and differs from the discussed Cu²⁺-centered polyhedra. The Cu3 position is 74% occupied by Cu⁺ cations and 26% occupied by Na⁺ cations. The Cu3-site atoms are coordinated by the two nearest O atoms, with the distance between the Cu3 and O2 of 2.032(2) Å and Cl⁻ anions, one at a distance of 2.394(5) Å and two at significantly larger distance of 3.1875(1) Å, resulting in trigonal bipyramidal coordination. The P–O bond lengths within the tetrahedron at the m plane range from 1.530(2) to 1.544(2) Å. The Na atom on the 2-fold axis is surrounded by eight ligands, i.e., six O anions with Na–O distances varying from 2.309 (1) to 2.886(2) Å and two Cl atoms at 3.270(2) Å (Figure 5a). This eight-fold coordination of the Na atoms is clearly confirmed via bond valence calculation (

Table 6. The bond valence calculations for Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl *.

	Cu1	Cu2	Cu3 **	P	Na	∑
O1		0.520		1.212	0.155 ×2↓ ×2→	2.04
O2			0.182 ×2↓	1.251	0.212 ×2↓ ×2→	1.98
O3	0.626 ×2↓ ×2→	0.534 ×2→				2.32
O4	0.212 ×4↓	0.395 ×2↓		1.213 ×2↓	0.064×2↓	1.88
Cl		0.209 ×2→	0.144 0.02 ×2↓ ×2→		0.073 ×2↓ ×4→	1.04
∑	2.10	2.05	0.95	4.89	1.01

* Symbols ^×2↓ and ^×2→ indicate the multiplication of the valence contribution along the column or row according to the symmetry. ** Calculated as occupied by 74% Cu⁺ and 26% Na⁺.

).

In the crystal structure of the compound Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl, Cu²⁺–centered polyhedra form ribbons, in which each CuO₆ octahedron shares four edges with four CuO₄Cl pyramids (Figure 4b). Likewise, each pyramid shares three edges, one edge with the neighboring CuO₄Cl polyhedron and two edges with the CuO₆ octahedra. The Cl vertex is common to two Cu2O₄Cl polyhedra, and the oxygen ligand O4 is common to four Cu polyhedra (two CuO₄Cl and two CuO₆). Thus, two types of Cu²⁺-centered polyhedra form condensed zigzag ribbons elongated in the direction of the c-axis of the unit cell. These ribbons are arranged along the midpoints of the lateral faces of the unit cell in accordance with the base-centered lattice (oS). Orthophosphate tetrahedra connect these ribbons into a three-periodic heteropolyhedral framework (Figure 5). Similar ribbons are described in the crystal structures of the mineral aleutite ((M₀.₅Cl)[Cu₅O₂(AsO₄)(VO₄)]) [21] and the synthetic phase ((Na,Li)₃(Cl,OH)[Cu₃OAl(PO₄)₃]) [22].

Each PO₄ group shares three oxygen vertices with Cu²⁺-centered polyhedra; hence, the PO₄ tetrahedron connects two neighboring copper ribbons along the a-axis. The fourth oxygen vertex of the PO₄ group is directed inside the framework channels and involved in the coordination of Na⁺ and Cu⁺ ions. The presence of these monovalent cations inside the large channels compensates for the negative charge of the heteropolyhedral framework.

3.4.2. Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl in the Series of Compounds with Oxo-Centered Pyroxene-like Chains

As mentioned above, the Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl crystal structure is based on the chains of interconnected CuO₆ octahedra and CuO₄Cl pyramids, sharing edges. In these chains, neighboring Cu atoms are located at distances of 3.0264(3) Å (Cu1–Cu2) and 3.1382(6) Å (Cu2–Cu2) from each other. The oxygen ligand O4 in each of these chains is exclusively surrounded by copper atoms, forming the μ₄ configuration.

According to the established nomenclature, copper oxo-salts with “additional” oxygen ligands (not shared with anionic complexes) are called oxo-cuprates. They are often described within the anion-centered approach [23,24]. In the case of the Na₂Cu₃O(Cu_0.8Na_0.2)(PO₄)₂Cl, oxo-centered [OCu₄] tetrahedra form pyroxene-like chains [O₂Cu₆]_∝ (Figure 4c) that belong to the “zweier chain topological type”, according to the classification in [24]. In the structure (II), these chains, extended along the c-axis of the unit cell, are separated from each other by Na-, P-, and Cu⁺-centered polyhedra.

It is noteworthy that the Na₂Cu₃O(Cu_0.8Na_0.2)(PO₄)₂Cl is isostructural to the oxo-cuprate Na₂Cu₃OCu(PO₄)₂Cl, in the crystal structure of which there is no Cu⁺/Na⁺ isomorphous substitution [25]. Additionally, in the same family of electronically distinct copper phosphate chlorides, an oxo-cuprate arsenate Na₂Cu₃OCu(AsO₄)₂Cl was recently obtained [26]. This phase crystallizes in the lower symmetry space group Pnma, which is a subgroup of the Cmcm group inherent to phosphate.

Other structurally related phases, i.e., NaCu₃OCu(PO₄)₂Cl [27] and Cu_1,5Cu₃O(PO₄)₂Cl [26], only contain divalent copper cations. All discussed structures are formed from the same ribbon fragments of Cu²⁺-centered polyhedra, interconnected through anionic tetrahedra. The differences between the structures lie in variations in the channel content within the topologically identical crystal frameworks of the composition [(Cu₃O)(XO₄)₂]²⁻_∝, where X denotes P or As. In the Na₂Cu₃O(Cu₀.₈Na₀.₂)(PO₄)₂Cl crystal structure, the channels contain salt inclusions of the |Na₂(Cu^I₀.₈Na₀.₂)Cl|²⁺ composition. In the arsenate structure, there is a similar inclusion of|Na₂Cu^ICl|²⁺, whereas phases containing only divalent copper cations comprise |NaCu^IICl|²⁺ and |Cu^II₁.₅Cl|²⁺. The channels contents differ in the amount of sodium cations, as well as the atomic positions of the interstitial ions. Notably, similar salt inclusions, often found in host–guest structures, are also observed in oxo-cuprates synthesized through halide flux methods, such as Na₂Cu₃OCu(PO₄)₂Cl [25] and NaCu₃OCu(PO₄)₂Cl [27]. However, hydrothermal synthesis techniques are also suitable for the preparation of these compounds, as shown in [22,28].

It is remarkable that anion-centered oxo-cuprate chains ([O₂Cu₆]_∝) (Figure 4c) form not only the crystal structure of (II) but also the other synthetic compounds mentioned above. These oxo-centered fragments ([O₂Cu₆]_∝) are characteristic of minerals found in fumaroles, namely kamchatkite (KCu₃OCl(SO₄)₂) [29,30], chloromenite (Cu₉O₂(SeO₃)₄Cl₆) [31], vergasovaite (Cu₃O[(Mo,S)O₄SO₄]) [32], dokuchaevite (Cu₂[Cu₆O₂](VO₄)₃Cl₃) [33], yaroshevskite (Cu₃[Cu₆O₂][VO₄]₄Cl₂) [34], and cupromolybdite ([O₂Cu₆][MoO₄]₄) [35]. Nevertheless, in the structures of minerals, tetrahedra forming oxo-centered [O₂Cu₆]_∝ chains have an orientation different to those in the structures of synthetic phases [26].

3.5. Mineralogically Probable Na₃V(OH)(HPO₄)(PO₄)

3.5.1. Description of Crystal Structure and Analysis of Interatomic Distances

In the crystal structure of the Na₃V(OH)(HPO₄)(PO₄) compound, V-centered octahedra share O2 vertices with the hydroxyl group to form chains of the 2₁ symmetry, parallel to the b axis of the unit cell. Each PO₄ tetrahedron shares two O vertices adjacent to the chain VO₄(OH)₂ octahedra, resulting in the arrangement of one-periodic mixed-type anionic fragments of [V(OH)(HPO₄)(PO₄)]³⁻. These negatively charged ribbons interconnect in a framework via Na⁺ ions and hydrogen bonds (Figure 6). The hydrogen bonds system includes the atom O1, which serves as a donor to form an O1—H1···..O6 bond of medium strength, in accordance with the D···A distance (

Table 7. The geometric characteristics of hydrogen bonds in the Na₃V(OH)(HPO₄)(PO₄) structure.

D–H···A	D–H, Å	H···A, Å	D···A, Å	D–H···A, °
O1–H1···O6	0.82(1)	1.80(2)	2.616(7)	172(9)
O2–H2···O1	0.82(1)	2.53(3)	3.319(7)	164(8)

). The O1 atoms also acts as an acceptor in the significantly weaker hydrogen bond O2—H2···..O1 (Figure 6b).

The basic structural units of the Na₃V(OH)(HPO₄)(PO₄) crystal structure are two symmetry-independent orthophosphate tetrahedra and a V³⁺O₄(OH)₂ octahedron, as shown in Figure 7. Both the phosphate tetrahedra with m symmetry exhibit significant distortion. The bond lengths in the P2O₄ tetrahedron vary from 1.508(5) to 1.558(3) Å (

Table 8. The interatomic distances (Å) in the crystal structure of Na₃V(OH)(HPO₄)(PO₄).

V-Centered Octahedron		P1–Tetrahedron		P2–Tetrahedron
V–O3 –O3 –O4 –O4 –O2 –O2 <V–O>	1.966(3) 1.966(3) 2.020(3) 2.020(3) 2.039(2) 2.039(2) 2.008	P1–O6 –O4 –O4 –O1 <P1–O>	1.526(5) 1.538(3) 1.538(3) 1.582(5) 1.544	P2–O5 –O7 –O3 –O3 <P2–O>	1.508(5) 1.510(5) 1.558(3) 1.558(3) 1.534
Na1–Octahedron		Na2–Polyhedron		Na3–Octahedron
Na1–O5 –O5 –O4 –O4 –O1 –O1 <Na1–O>	2.259(3) 2.259(3) 2.448(3) 2.448(3) 2.747(4) 2.747(4) 2.485	Na2–O7 –O5 –O2 –O4 –O4 <Na2–O>	2.175(6) 2.238(6) 2.435(5) 2.550(3) 2.550(3) 2.390	Na2–O3 –O3 –O6 –O6 –O7 –O7 <Na3–O>	2.427(3) 2.427(3) 2.460(4) 2.460(4) 2.482(4) 2.482(4) 2.456

). The shortened P2–O5 and P2–O7 bond lengths equal to 1.508(5) Å and 1.510(5) Å, respectively, serve to balance the valence charge on the corresponding oxygen atoms (

Table 9. The bond valence data for Na₃V(OH)(HPO₄)(PO₄).

	Na1	Na2	Na3	V	P1	P2	H1	H2	Σ
O1	0.086 ×2↓ ×2→				1.093		0.71	0.06	2.04
O2		0.163		0.449 ×2↓ ×2→				0.94	2.00
O3			0.166 ×2↓	0.547 ×2↓		1.167 ×2↓			1.88
O4	0.159 ×2↓	0.129 ×2↓		0.473 ×2↓	1.231 ×2↓				1.99
O5	0.235 ×2↓ ×2→	0.246				1.335			2.05
O6			0.155 ×2↓ ×2→		1.272		0.29		1.87
O7		0.28	0.148 ×2↓ ×2→			1.328			1.90
Σ	0.96	0.95	0.94	2.94	4.83	5.00	1	1

The symbols ^×2↓ and ^×2→ indicate the multiplication of the valence contribution along a column or row in accordance with symmetry.

). The P1O₃(OH) tetrahedron is characterized by a notably longer P1–O1 distance of 1.582(5) Å to the oxygen atom of the hydroxyl group compared to other P1–O bond lengths ranging from 1.526(5) Å to 1.538(3) Å. Both the average P–O distances associated with P1 and P2 tetrahedra are equal to 1.534 and 1.544 Å, which are usual for the orthophosphate groups (Table 8). The V³⁺ ion centers the centrosymmetric V³⁺O₄(OH)₂ octahedron characterized by V–O bond distances within the range 1.966(3)–2.039(2) Å, with an average value of 2.01 Å, which is typical for V³⁺ cations. The structure also contains three symmetrically independent sodium atoms, forming centrosymmetric octahedra Na1O₄(OH)₂ and Na3O₆, and a trigonal bipyramid Na2O₅. In these polyhedral, the Na–O distances vary from 2.175(6) to 2.550(3) Å, while the Na1–OH values are expectedly larger and equal to 2.747(4) Å.

The crystal structure of the compound with the chemical formula Na₃V(OH)(HPO₄)(PO₄) was first published in 2008 in [36], the authors of which used X-ray powder data in order to obtain the structure model. In our study based on a single-crystal X-ray diffraction experiment, the model was confirmed, and hydrogen atoms were also located and their positions were refined. Moreover, our structural study provides precise data regarding the geometry of PO₄ tetrahedra, which allows us to exclude disorder in P-sites, as supposed in [36].

It is notable that the compound Na₃V(OH)(HPO₄)(PO₄) has isostructural analogues with V³⁺ substituted for Al³⁺ or Ga³⁺ [37], which, together, form a group of compounds with the general formula Na₃(M)(OH)(HPO₄)(PO₄), where M = V, Al, or Ga.

Similar crystal chemical properties of Fe³⁺ and V³⁺ ions determine a widespread distribution of vanadium in the magmatic process. Thus, iron is a kind of solvent of trivalent vanadium, causing its dispersed state in igneous rocks. In hydrothermal solutions in a reducing environment, vanadium migrates in the form of (V⁴⁺O)²⁺ or V³⁺ ions, which are mainly precipitated as part of hydrated silicate minerals, such as dimorphic varieties of cavansite and pentagonite, i.e., Ca(V⁴⁺O)Si₄O₁₀·4H₂O. Though the (V⁴⁺O)²⁺ or V³⁺ phosphates are rare, among them are sincosite (Ca(V⁴⁺O)₂(PO₄)₂·5H₂O), and bariosincosite, (Ba(V⁴⁺O)₂(PO₄)₂·4H₂O), and a unique V³⁺ representative known as springcreekite (BaV³⁺₃(PO₄)(PO₃OH)(OH)₆). Both springcreekite and bariosincosite, found in a small vein deposit formerly mined for Cu, are considered to be formed via subaerial and near surface aqueous alteration at low temperatures [38,39]. It is remarkable that sincosite and bariosincosite synthetic analogues have been obtained in the laboratory under similar P/T conditions [40,41,42]. The geochemically reasonable system for Na₃V³⁺(OH)(HPO₄)(PO₄) synthesis at 270 °C and 70 atm, as well as the discovery of natural V³⁺ phosphate springcreekite, indicate the mineralogical probability of this compound.

3.5.2. Magnetic Properties of Na₃V(OH)(HPO₄)(PO₄)

V³⁺ has a 3d² electronic configuration. The low value of spin S = 1 in combination with the reduced dimensionality of the magnetic subsystem can be a source of quantum behavior at low temperatures. Previously, the magnetic behavior of Na₃V(OH)(HPO₄)(PO₄) was studied in the temperature range of 50–250 K. Magnetic susceptibility measurements evidenced sizable exchange interactions in the title compound. Therefore, it was interesting to study its behavior down to lower temperatures.

Magnetization M was measured using a Quantum Design automated PPMS-9T (Physical Property Measurement System) equipped with a Vibrating Sample Magnetometer (VSM). The temperature dependences of the magnetic susceptibility were taken in field-cooled (FC) and zero-field-cooled (ZFC) regimes at B = 0.1 T in the temperature range of 2–300 K. Through the temperature region, ZFC and FC curves almost perfectly coincided (Figure 8). At elevated temperatures, the χ(T) curves followed the Curie–Weiss law with the addition of the following temperature independent term χ₀:

$$\chi ={\chi }_0+\frac{C}{T-\mathsf{\Theta }}$$

(1)

The fitting of the FC curve in the range 200–300 K results in the Curie constant C = 0.99 emu K/mol and Weiss temperature Θ = −35 K. The value of C corresponds to the presence of V³⁺ ions in the structure using the g-factor g = 1.99. The negative Weiss temperature Θ suggests antiferromagnetic correlations between V³⁺ cations. The right inset in Figure 8 presents inverse magnetic susceptibility, showing deviation from linearity near 10 K. The extrapolation of χ⁻¹(T) curve to negative temperatures indicates a slightly lower value of Weiss temperature, established via the approximation of the high-temperature region of the χ(T) curve.

The magnetization curve (left inset in Figure 8) reveals that at T = 2 K and in a magnetic field of B = 9 T, the magnetization is 0.78 μ_B. This value is significantly lower than that of the saturation magnetization, calculated using the following formula: M_sat = ngSμ_B, which is equal to 2 μ_B.

According to the slightly elongated geometry of VO₄(OH)₂ polyhedra, two d electrons of V³⁺ occupy two out of three t_2g orbitals. The established presence of antiferromagnetic exchange interactions between V-centered octahedra within chains is supported by the bridge angle V─O─V = 127.5° obtained from structural data. The absence of evidence for low-dimensional behavior in magnetic susceptibility measurements allows us to suggest that the values of exchange magnetic parameters of intrachain and interchain interactions are comparable in Na₃V(OH)(HPO₄)(PO₄). Lower temperatures are needed to achieve a long-range order in the title compound.

4. Conclusions

In our search for the compounds with alkali and transition metals, three mineral-like phases were synthesized in the form of single crystals: silicate Na₂CoSiO₄, a structural analogue of liberite, and two phosphates, namely Na₂Cu₃O(Cu,Na)(PO₄)₂Cl and Na₃V(OH)(HPO₄)(PO₄). Their crystal structures were refined (partially based on low temperatures) via X-ray diffraction, unlike some earlier studies performed using the powder samples. Our research, which generally confirmed previous results, allowed us to obtain more precise atomic coordinates, distances, and hydrogen bonds. It was shown that the silicate Na₂CoSiO₄ is characterized by a strongly pseudo-orthorhombic lattice and a monoclinic crystal structure, refined using a pseudomerohedric microtwin. It was established that in the crystal structure of Na₂Cu₃O(Cu,Na)(PO₄)₂Cl, the Cu⁺ position is partially substituted by 26% Na atoms, in contrast to previous structural data. We also highlighted the original crystal–chemical interpretation of the structural features and properties of Na₂CoSiO₄ and Na₂Cu₃O(Cu,Na)(PO₄)₂Cl and the studied magnetic behavior of Na₃V(OH)(HPO₄)(PO₄).