Synthesis, Structure, and Physical Properties of RbCr₂Se₂O

Abstract

Layered compounds containing the T₂O plane (T = transition metal), which is the anti-type of the CuO₂ plane in cuprate superconductors, have been explored widely because of their diverse physical properties. Among them, KV₂Se₂O has attracted much attention due to its interesting physical properties, especially the magnetic order. In this work, we report a new isostructural chromium oxyselenide, RbCr₂Se₂O. It was synthesized using a solid-state method using Rb₂CO₃ as the source of Rb and O for the title compound, with the assistance of Ba. The compound crystallizes in the space group P4/mmm with lattice parameters a = 4.01123(8) Å and c = 7.49357(18) Å. Magnetic susceptibility measurements indicate an antiferromagnetic transition at 345 K for RbCr₂Se₂O and also above room temperature, as the Néel temperature is T_N ≈ 400 K for KV₂Se₂O. The analysis of variable temperature XRD data reveals the anisotropic thermal expansion of the RbCr₂Se₂O lattice. The almost unchanged lattice parameter a near the transition temperature and the broad peak with an onset temperature of ~360 K in the differential scanning calorimetry data may have a relationship with the magnetic ordering. The measurement of electrical resistivity demonstrates the semiconducting behavior of RbCr₂Se₂O. The thermal activation model and variable-range hopping model are proposed to describe the conduction mechanism in the high- and low-temperature ranges, respectively.

1. Introduction

Layered compounds containing 3d transition metals have attracted great attention due to their diverse structures and fascinating properties. The species of 3d transition metal always plays a key role in physical properties. For example, superconductivity can be induced in cuprates with CuO₂ layers [1] and iron-based compounds with FeAs/FeSe(S) [2,3,4] planar layers in a square lattice. The AT₂Q₂O compounds (A = K, Rb, Cs, Ba, (LaO)₂, etc.; T = 3d transition metal; Q = S, Se, As, Sb, etc.) crystallized in tetragonal lattices form a rich family, which all own T₂O planes with an anti-CuO₂ structure in T₂Q₂O layers. BaTi₂Sb₂O [5] and Ba_1−xNa_xTi₂Sb₂O [6] were discovered as superconductors with transition temperatures of T_c = 1.2 K and 5.5 K, respectively. (LaO)₂Mn₂Se₂O, (CeO)₂Mn₂Se₂O, and (PrO)₂Mn₂Se₂O were reported to have antiferromagnetic ordering with transition temperatures T_N = 164–184 K and undergo a structural distortion relating to the static displacement of the oxide ions out of the square planar [Mn₂O]²⁺ layer [7]. (LaO)₂Fe₂Se₂O [8] and (LaO)₂Co₂Se₂O [9] showed antiferromagnetism and Mott-insulating behavior. (LaO)₂Ni₂Se₂O was proposed as a potential candidate for superconductivity on doping or applying pressure [10]. CsV₂Se₂O was reported as a semiconductor exhibiting a density wave-like transition at 168 K. The isostructural CsV₂Se_1.5O, with the deficiency of Se, tended to behave like a metal. V₂Se₂O was obtained by extracting the Cs ions from the structure and became a strong correlated insulator [11]. Similar van der Waals layered materials V₂Te₂O [12] and Ti₂Te₂O [13] were synthesized through deintercalation based on their alkali metal-containing parent compounds. Recently, KV₂Se₂O was reported as a metallic room-temperature altermagnet with d-wave spin-momentum locking [14]. But another study demonstrated that KV₂Se₂O exhibited a G-type antiferromagnetic structure with a Néel temperature of T_N ≈ 400 K, and altermagnetism was absent in bulk KV₂Se₂O [15]. Among the numerous AT₂Q₂O compounds, the chromium-based counterparts are still rare [16]. Cr₂As₂O layers were found in Sr₂CrO₂Cr₂OAs₂, showing a long-range antiferromagnetic ordering below 590 K. Attempts had been made to introduce Cr onto the T site of (K/Rb/Cs)T₂Q₂O but failed [13]. Monolayer Cr₂Se₂O and Cr₂S₂O were proposed as potential two-dimensional altermagnets and half-metals in a first-principles study [17].

In this work, RbCr₂Se₂O was first obtained as a new member of the AT₂Q₂O family. Based on the reported methods for (K/Rb/Cs)T₂Q₂O compounds, minor adjustments were made to synthesize polycrystalline RbCr₂Se₂O successfully. The compound is an antiferromagnetic semiconductor. Anisotropic thermal expansion of the layered crystal structure is observed. Around the antiferromagnetic transition temperature, anomalies are observed in the temperature dependence of lattice parameters a and c, as well as a hump in the differential scanning calorimetry data, which may be relevant to the magnetic ordering.

2. Materials and Methods

Polycrystalline samples of RbCr₂Se₂O and the CrSe precursor were synthesized via conventional solid-state methods. First, for the preparation of CrSe, 4 g of a mixture containing powdered elements Cr (99.99%, Alladin, Shanghai, China) and Se (99.99%, Alladin, Shanghai, China), weighted according to the stoichiometric ratio, was ground and pressed into a pellet. The pellet was sealed in an evacuated quartz tube and heated at 800 °C for 20 h. Then, 0.2915 g of Rb₂CO₃ powder (99.99%, Alladin, Shanghai, China) and 0.6295 g of CrSe powder with a molar ratio of 1.05:4 were mixed homogeneously and pressed into a pellet. The pellet was loaded into the Al₂O₃ crucible. A total of 0.2 g of Ba pieces was cut off from a Ba rod. The molar ratio of Ba to Rb₂CO₃ was 1.2:1. As shown in Figure S1c, both the Ba pieces and the Al₂O₃ crucible containing the pellet of Rb₂CO₃ and CrSe were placed in the same quartz tube, keeping the Ba pieces inside the quartz tube and outside the Al₂O₃ crucible, ensuring no direct contact between the pellet and Ba pieces. The tube was evacuated and heated to 800 °C, kept for 24 h, and cooled to room temperature. Finally, the polycrystalline sample of RbCr₂Se₂O was obtained, which was dark black and air-sensitive. In the ideal situation, the 5% excess of Rb₂CO₃ leads to a 5% excess of Rb₂O and 5% excess of CO₂, which should be left in the pellet and absorbed by Ba, respectively. Rb₂O is highly reactive towards water and it is consumed during the short-time exposure to the air when sampling for X-ray diffraction (XRD). So, no discernible reflection due to Rb₂O was found in XRD, maybe because of the small amount, bad crystallinity, formation of other phases, such as RbCr₅Se₈, or reaction with H₂O or other things during synthesis and measurement operations. The Ba pieces after reaction were collected and weighed. The actual mass increment was 0.0324 g, 0.0029 g lighter than the calculated mass based on the supposed reactions in Section 3. All sample preparations were performed inside a glovebox with a controlled Ar atmosphere (Labmaster, O₂ and H₂O < 1 ppm).

Room-temperature powder X-ray diffraction data were collected using a PANalytical X’Pert PRO diffractometer (Cu Kα radiation, Malvern Panalytical, Almelo, The Netherlands) with a graphite monochromator in reflection mode (2θ = 10–120°, step 0.02° (2θ)). Rietveld refinements were performed with the Fullprof software suite (Version 2.0 August 2008) [18]. The XRD sample was prepared in a glovebox. The collection of XRD patterns was conducted in the air, lasting 10 min for one XRD pattern. The variable-temperature powder XRD patterns were collected (2θ = 10–120°, step 0.02° (2θ)) in an Ar atmosphere. A constant temperature was maintained for more than half an hour before data collection in the setting temperature range of 300–380 K.

The bond valence sum (BVS) method can be used to estimate the oxidation states of metal ions in coordination compounds [19]. For atom i, its oxidation state V_i is given as V_i = Σs_ij = Σ{exp[(r₀-d_i-j)/B]}, where s_ij is the valence of the bond between atoms i and j, d_i-j is the bond length between atoms i and j, and r₀ and B are empirically determined parameters. B was set to be 0.37 [19] and r₀ was 2.29 Å for the Cr-Se bond [20]. For RbCr₂Se₂O, atom Cr is coordinated with six atoms, including two O atoms and four Se atoms. The Cr-Se distance and Cr-O distance are d_Cr-Se = 2.6542(10) Å and d_Cr-O = 2.0056(4) Å. So the oxidation state of Cr is V_Cr = 4 × exp[(r₀-d_Cr-Se)/B] + 2 × [exp(r₀-d_Cr-O)/B]. Taking r₀ = 1.73 Å for the Cr²⁺-O bond and r₀ = 1.724 Å for the Cr³⁺-O bond [20], the oxidation state of Cr is +2.44 and +2.43, respectively.

Magnetic susceptibilities were measured using a vibrating sample magnetometer (VSM, Quantum Design, San Diego, CA, USA). For magnetic measurements, samples were sealed in a capsule in a glovebox and then transferred from the glovebox to the equipment. Electrical resistivity measurements were performed by a standard four-probe method on a physical property measurement system (PPMS-9T, Quantum Design, San Diego, CA, USA). The differential scanning calorimetry (DSC) measurement was performed using a NETZSCH STA 449 F3 Jupiter 21 thermal analyzer (NETZSCH, Selb, German). The sample was heated from room temperature to 1273 K under a dynamic atmosphere of Ar (flow rate: 100 mL/min) with a controlled heating rate of 5 K min⁻¹. Approximately 10–15 mg of the powdered sample was placed in an Al₂O₃ crucible, and an empty Al₂O₃ crucible served as a reference. The time of exposure to air was less than 3 min in the measurement operation.

3. Results and Discussion

Figure 1 shows the experimental, calculated, and difference powder X-ray diffraction (XRD) profiles of the RbCr₂Se₂O polycrystalline sample. All reflections were well-indexed based on the tetragonal crystal structure with the space group P4/mmm. No peaks due to impurities were detected. Room-temperature structural parameters obtained from the Rietveld refinement are presented in

Table 1. Crystal data and structural refinement parameters for RbCr₂Se₂O at 300 K.

Parameter	Value
Compound	RbCr2Se2O
Space group	P4/mmm
a (Å)	4.01123(8)
c (Å)	7.49357(18)
ρ (g cm−3)	5.005
Rp (%)	4.29
Rwp(%)	5.59
GoF	2.60

and

Table 2. Details of the crystal structure of RbCr₂Se₂O.

Atom	Site	x	y	z	Occupancy	Biso
Rb	1b	0	0	0.5	1	1.21(5)
Cr	2f	0.5	0	0	1	0.72(5)
Se	2h	0.5	0.5	0.2320(2)	1	0.79(4)
O	1a	0	0	0	1	2.2(3)

, and the corresponding parameters of the titanium analog RbTi₂Se₂O and the vanadium analog RbV₂Se₂O are displayed in Table S1. RbCr₂Se₂O has Cr₂O planes, which have an anti-CuO₂ structure typical in copper-based superconductors. Se atoms are located directly above and below the center of the Cr₂O squares, together forming the Cr₂Se₂O layers, which are separated by Rb atomic layers.

Preliminary air sensitivity was measured by exposing the sample to the air. As shown in Figure 2a, four XRD patterns were displayed with different air exposure times. Each pattern was collected for 10 min. The air exposure time was set as the time interval between the start time of exposure to air and the midpoint time of XRD pattern collection. Peak broadening and the appearance of new peaks, such as the one in the figure, suggest RbCr₂Se₂O cannot remain stable in the air. As the air exposure time increases, the lattice parameter a decreases and lattice parameter c increases, both in a nonlinear way. The change in lattice parameter a and c may be caused by the deintercalation of Rb [13].

The value of the Cr-O distance is 2.0056(4) Å, equal to half the lattice parameter a of RbCr₂Se₂O. Similar values have been reported [21,22,23]. The Cr-O distance within the CrO₂ square-planar sheets of Sr₂CrO₂Cr₂As₂ is 2.00400(1) Å [21]. In Sr₂CrO₂Cr₂OAs₂, there are two unique Cr sublattices, one containing Cr²⁺ in a CrO₄As₂ environment and the other Cr³⁺ in CrO₂As₄ coordination. The Cr-O distance in the CrO₂As₄ coordination is 2.02245(4) Å [23]. The oxidation state of Cr differs from +2 to +6 and the Cr-O distance spans from ~1.65 Å to ~2.05 Å [24]. Both the oxidation state and the coordination number of chromium have an effect on the Cr-O distance, with the former playing a predominant role. The chromium ion, with a lower valence, usually has a longer Cr-O distance [19,24]. The oxidation state of Cr is evaluated by its bond valence sum (BVS) here [19].The nominal valence of Cr in RbCr₂Se₂O is +2.5, between +2 and +3, so both values of r₀ for Cr²⁺ and Cr³⁺ are used for calculation. The obtained Cr BVS value is +2.44 for Cr²⁺and +2.43 for Cr³⁺, indicating no significant difference and consistency with the expected value. Further analysis by X-ray absorption spectroscopy may be needed to verify the result [25,26]. Since there is only one crystallographic site for Cr, this result indicates an electron delocalization or a mixed distribution of Cr²⁺and Cr³⁺. The Cr-Se distance with a value of 2.6542(10) Å is larger than corresponding values of 2.54–2.59 Å for Cr³⁺ [27,28,29,30]. Compared with the Cr-Se distance of 2.60542 Å in the CrSe₆ octahedron observed in CrSe (Crystallography Open Database ID 9008890), the Cr-Se distance in RbCr₂Se₂O is slightly elongated to 2.6542(10) Å as compensation for the introduction of two shorter Cr-O bonds into the CrSe₄O₂ octahedron.

Stabilizing the low oxidation state for Cr in oxides increases the difficulty in sample syntheses, frequently accompanied by impurities [21,22]. Similar situations also occur with vanadium oxyselenides [11,13] and our work. RbCr₂Se₂O appears to be in a complex phase space near other competing compounds which are still awaiting discovery. We found that unknown phases appeared as byproducts of RbCr₂Se₂O, with typical peaks at 8.6° in the XRD patterns, as shown in Figure S2 in the Supplementary Materials. These peaks could be suppressed effectively by 5% excess Rb₂CO₃. In 2016, polycrystalline CsV₂S₂O was obtained through a solid-state reaction between stoichiometric amounts of a mixed cesium oxide Cs₂O_1.3, VS, S, and V₂O₅ [31]. The cesium oxide precursor was prepared from Cs and O₂ in a complex way. Based on the relative composition of Cs₂O and Cs₂O₂ determined by XRD, the nominal formula was determined as Cs₂O_1.3. In the following studies, alkali metals, instead of alkali metal oxide, and 3d transition metal oxides were commonly used as starting materials to obtain the isostructural analogs [11,13]. Still, this method did not work in the synthesis of RbCr₂Se₂O [13]. In this work, the most significant difference in the synthesis method is using Rb₂CO₃ as the source of Rb and O for RbCr₂Se₂O, shown in Figure S1. Meanwhile, the addition of Ba was proven to play an indispensable role in success.

How RbCr₂Se₂O was formed by the reactions between CrSe, Rb₂CO₃, and Ba was confusing. We have tried to explain the reaction process here and the supposed reactions are displayed in Figure S1d. Ba pieces and the pellet of a mixture of CrSe powder and Rb₂CO₃ powder were placed separately in a vacuumed silica tube before the reaction, as shown in Figure S1c. As the tube was heated to 800 °C, Rb₂CO₃ decomposed to produce Rb₂O and CO₂, the reaction in yellow background color in Figure S1d. Rb₂O was needed but only provided half the amount of oxygen for RbCr₂Se₂O. The other half of the oxygen was carried by CO₂ gas. CO₂ then reacted complicatedly with excess Ba [32], the reaction in green background color in Figure S1d. Although the reaction process was not explicit, the result showed that these reactions released oxygen from CO₂, which RbCr₂Se₂O needed, the reaction in red background color in Figure S1d, and turned Ba into BaO, barium carbides, and other possible phases. The Ba pieces became crispy and showed white and dark gray colors. These pieces were collected and examined by XRD. Only BaO was identified, leaving some undefined peaks. In this synthesis route, the absence of Ba would lead to complete failure. As demonstrated in Figure S3, no RbCr₂Se₂O could be found in the mixture. Most of the starting materials were consumed, and only a small amount of Rb₂CO₃ remained after the reactions. The product contained, but was not limited to, Cr₂O₃, RbCr₅Se₈, Cr₃Se₄, Rb₂Se₅, and Rb₂CO₃.

An antiferromagnetic transition at 345 K was identified in RbCr₂Se₂O. When it comes to magnetic measurements for antiferromagnetic materials, it has to be mentioned that a small level of contamination by ferromagnetic/ferrimagnetic impurities will seriously disturb the magnetic signals [22]. When exploring the magnetic property of RbCr₂Se₂O, the magnetization of one sample could be several times greater than another one, affected by random amounts of impurities with a strong magnetic response, as shown in Figure S4. There are some anomalies below 200 K in the magnetic susceptibility curves displayed in Figure S4a, but it is hard to distinguish between intrinsic transitions and signals of impurities. However, all magnetic susceptibility curves shared the same anomaly around 345 K which was sometimes barely discernible in the ferromagnetic background. The anomaly was probably intrinsic. Figure 3a shows the temperature-dependent magnetic susceptibility for RbCr₂Se₂O, measured at a magnetic field of 1 T with field-cooling mode. With decreasing temperature, the magnetic susceptibility increases from 380 K to 345 K. Then the magnetic transition occurs at 345 K, accompanied with a drop in magnetic susceptibility, suggestive of an antiferromagnetic transition. A field-dependent magnetization curve gives a consistent result, exhibiting a linear relationship between magnetization and the applied magnetic field, as displayed in the inset of Figure 3a. The small deviation from the straight line arises from minor impurities. Differential scanning calorimetry (DSC) measurement was conducted to explore this transition. A previous study suggests materials with low self-magnetization and low magnetic susceptibility values cannot show any noticeable DSC changes for a magnetic transition [33]. For RbCr₂Se₂O, no change is found at 345 K in DSC data but a broad hump is observed, as shown in Figure 3b. By subtracting the baseline, a peak with an onset temperature of 360 K is demonstrated in the inset of Figure 3b. Unlike sharp peaks induced by first-order transition, this peak has a second-order transition-like profile and may be correlated with the antiferromagnetic transition.

Variable-temperature XRD is performed to explore the evolution of the crystal structure of RbCr₂Se₂O in the temperature range of 300–390 K. No new diffraction peaks indicating a structural phase transition are observed over the temperature range. The temperature dependence of the lattice parameters a and c and the c/a ratio obtained via Rietveld refinement of the powder XRD data are demonstrated in Figure 4. As temperature increases from 300 K to 330 K, the lattice parameter a increases in a linear-like way. The coefficient of thermal expansion is α_a = da/(a_300KdT) = 3.45 × 10⁻⁶ K⁻¹ (coefficient of determination R² = 0.8424), where a_300K is the value of a at 300 K. In the temperature range of 330 K to 390 K, there is no obvious monotonic trend of change, indicating a possible zero coefficient of thermal expansion. The lattice parameter c and the ratio of c/a increase with increasing temperature. Treated in the same way as for the lattice parameter a, the coefficient of thermal expansion of lattice parameter c is α_c = dc/(c_300KdT) = 16.8 × 10⁻⁶ K⁻¹ (coefficient of determination R² = 0.9987) and α_c = dc/(c_330KdT) = 22.3 × 10⁻⁶ K⁻¹ (coefficient of determination R² = 0.9985) for the temperature range of 300–330 K and 330–390 K, respectively. The coefficient α_c is about 4.9 times greater than α_a in the temperature range of 300–330 K, suggesting the anisotropic thermal expansion of the RbCr₂Se₂O crystal structure. The Cr-O distance d_Cr-O, equal to half the lattice parameter a, changes in the same way, as shown in Figure 4a. The Cr-Se distance d_Cr-Se (shown in Figure 5b) and the angle θ_Se-CrSe (shown in Figure 5c) are basically unchanged with increasing temperature, while the Rb-Se distance d_Rb-Se (shown in Figure 5d) is increased by more than 0.04 Å. In the warming process, it is easy for the weak interlayer ionic Rb-Se bonding to be elongated, becoming the main contribution to the increase in the c axis. Meanwhile, the Cr₂Se₂O layers containing covalent Cr-O bonds and Cr-Se bonds remain robust. In magnetic materials, magnetic ordering offers a negative contribution to the thermal expansion, opposite to the positive contribution of phonons, leading to abnormal thermal expansion and even negative thermal expansion [34]. It is reported that, in antiferromagnetic CrSb, negative thermal expansion is induced by the antiferromagnetic ordering of the Cr atom over a broad temperature window (360–600 K) when the magnetic moment is reduced significantly [35]. The change in α_a may be influenced by the antiferromagnetic transition in a similar way in RbCr₂Se₂O. Further measurements are needed to reveal the relationship between the magnetic transition and thermal expansion.

Figure 6a shows the temperature-dependent resistivity ρ(T) curve of a pressed tablet of polycrystalline RbCr₂Se₂O measured in the absence of an externally applied magnetic field. With decreasing temperature, the resistivity increased rapidly from 1.4 Ω cm at 380 K to 1.9 × 10⁴ Ω cm at 10 K, indicating the semiconducting behavior of RbCr₂Se₂O. The thermal activation model (lnρ ∝ 1/T), the three-dimensional variable-range hopping (VRH) model (lnρ ∝ T ^−1/4), and the small-polaron hopping (SPH) model [ln(ρ/T) ∝ 1/T)] were tried to fit the temperature dependence of resistivity in the whole temperature range of 10–380 K, but all failed. Two mechanisms of charge transfer were used for different temperature intervals, 10–70 K and 250–380 K, chosen based on a better coefficient of determination (R²) of the fitting. In the high-temperature interval of 250–380 K, the resistivity curve can be explained by the thermal activation model ρ = ρ₀exp (E_a/k_BT), where ρ₀ is a constant prefactor, E_a is the activation energy, and k_B is Boltzmann’s constant. lnρ shows a linear relationship with T⁻¹, and the corresponding fitting line is shown in Figure 6b. The fitted activation energy, E_a, is approximately 51 meV (coefficient of determination R² = 0.9995). When the temperature is low, the behavior of charge carriers may be influenced by the disordered distribution of Cr²⁺ and Cr³⁺ on a single crystallographic site [36]. The VRH model describes a low-temperature situation when the thermal energy is not high enough for charge carriers to hop to their nearest neighbors. Hence, carriers need to hop further to a site with a smaller potential difference. In the model ρ = Aexp[(T₀/T)^1/4], A is a prefactor, and T₀ is the characteristic temperature. As demonstrated in Figure 6c, lnρ versus T^−1/4 exhibits a linear behavior, and the characteristic temperature T₀ = 5 × 10⁵ K (coefficient of determination R² = 0.9995) can be obtained from the slope of lnρ versus T^−1/4. The magnitude of T₀ can be used as an indicator of the VRH phenomenon, and the relatively low value of T₀ compared with those in the scale of 10⁶–10⁸ suggests that RbCr₂Se₂O is not a very strong disordered or localized system [37,38,39,40].

4. Conclusions

In conclusion, RbCr₂Se₂O is reported here as the first chromium oxychalcogenide in the AT₂Q₂O system. It was successfully synthesized via a conventional solid-state method, taking RbCO₃ and CrSe as starting materials and introducing Ba to balance the reaction. RbCr₂Se₂O crystallizes in a tetragonal crystal structure containing Cr₂Se₂O layers separated by Rb ions. No structural transition was found over the temperature range of 300–390 K. RbCr₂Se₂O exhibited anisotropic thermal expansion in the crystal structure, driven by the large strength difference between the weak interlayer ionic bond and the robust covalent bonds in the Cr₂Se₂O layers. Magnetic and electrical property measurements revealed that RbCr₂Se₂O is a semiconductor with an antiferromagnetic transition at 345 K. Both the temperature dependence of lattice parameters and differential scanning calorimetry data showed anomalies that may be associated with the magnetic ordering. Resistivity at low temperatures was described by the three-dimensional variable-range hopping mechanism. Monolayer Cr₂Se₂O is predicted as a two-dimensional altermagnet by theoretical calculation, and the van der Waals phase Cr₂Se₂O may be obtained by extracting Rb⁺ ions from the parent compound RbCr₂Se₂O. Hence, RbCr₂Se₂O offers an opportunity for further exploration of altermagnets.