Crystal structure and magnetic properties of Cr 3 Te 5 O 13 Cl 3 †

A new chromium tellurite oxochloride, Cr3Te5O13Cl3, has been prepared by solid-state reaction and the crystal structure was determined by single crystal X-ray diffraction. The compound crystallizes in the noncentrosymmetric space group P212121 with the unit cell a = 4.90180(10) Å, b = 17.3394(2) Å, c = 17.5405(2) Å, Z = 4, R1 = 0.0282. The Cr 3+ ions have octahedral [CrO6] oxygen coordination, the Te 4+ ions have one sided [TeO3] and [TeO3Cl] coordinations. The [CrO6] octahedra are edge sharing and form chains extending along [100]. These are connected by corner sharing [TeO3] and [TeO3Cl] groups to form layers parallel to (110). The layers are connected by weak interactions in between Te in the layers and Cl ions located in between. The compound undergoes antiferromagnetic ordering at ∼34 K with a Weiss constant of −230 K. Isothermal magnetization measurements reveal a critical field of about 0.25 T above which the magnetization versus field changes from linear to a Brillouin-like saturation behaviour. The frustration ratio amounts to ∼6.8 indicative of sizable competing antiferromagnetic spin-exchange interaction. The dielectric constant ε (6 kHz) amounts to ∼7.9 and decreases by about 1% on cooling from 50 K to liquid helium temperatures.


Introduction
Oxohalide compounds comprising p-block lone-pair elements such as Se 4+ , Te 4+ , or Sb 3+ often exhibit unusual structural and physical properties because both the stereochemically active lone-pair and the halide ion contribute to open up the crystal structures by terminating structural elements and by occupying space in the nonbonding regions.The nonbonding side of the lone-pair elements naturally faces other lone-pair elements or halide ions to generate extended nonbonding volumes.This may lead to formation of (i) layered compounds with only weak van der Waals interactions in between the layers, as found in e.g.][12] The majority of compounds containing Cr 3+ are oxides and CrOCl is the only oxo-chloride previously reported, plus some hydrates; e.g.4][15][16] Only one tellurite containing Cr 3+ is previously described in the literature; Cr 2 Te 4 O 11 . 17To the best of our knowledge the present compound Cr 3 Te 5 O 13 Cl 3 is the first chromium oxo-chloride comprising a p-element lone pair cation.

Experimental
The new compound Cr 3 Te 5 O 13 Cl 3 was synthesized via chemical reactions in sealed evacuated silica tubes.Needle shaped single crystals were obtained from a mixture of TeO 2 (Sigma-Aldrich 99+%), Cr 2 O 3 (Sigma-Aldrich) and CrCl 3 (Sigma Aldrich) in the molar ratio 3 : 1 : 1.The reaction was carried out in a muffle furnace at 500 °C for 1 month.The product was a mixture of green crystals of the title compound and unreacted starting material.Single crystals of the title compound did grow on top of the powder so it was easy to separate them.† Electronic supplementary information (ESI) available: Tables of atomic coordinates and equivalent isotropic displacement parameters, bonding distances and angles, and Bond Valence Sum (BVS) calculations.Further details on the crystal structural investigations can be obtained from the Fachinformationszentrum Karlsruhe, Abt.PROKA, 76344 Eggenstein-Leopoldshafen, Germany (fax +49-7247-808-666; E-mail: crysdata@fiz-karlsruhe.de) on quoting the depository number CSD 425829.See DOI: 10.1039/c3dt50706h a Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden.E-mail: iwan.zimmermann@mmk.su.se, mats.johnsson@mmk.su.se;Fax: +46 8 152187; Tel: +46 8 162169 Single crystal X-ray diffraction experiments were carried out on an Oxford Diffraction Xcalibur3 diffractometer equipped with a graphite monochromator.The data collection was carried out at 293 K using MoKα radiation, λ = 0.71073 Å.Data reduction was done with the software CrysAlis RED that was also employed for the analytical absorption correction. 18The structure solution was carried out with SHELXS and the refinement with SHELXL 19 in the WINGX 20 environment.Restraints were applied to oxygen O5 and O13 to refine their anisotropic temperature parameters to positive-definite values.Atomic coordinates and isotropic temperature parameters for all atoms are given in the ESI.† Structure data are reported in Table 1.Structure plots are made with the program DIAMOND. 21or the magnetic measurements crystals were separated manually and ground in an agate mortar before putting in a gelatine capsule.The magnetic susceptibilities of a powder compact of Cr 3 Te 5 O 13 Cl 3 have been measured under fieldcooling and zero field-cooling conditions with a PPMS Physical Property Measurement System and an MPMS Magnetic Property Measurement System (both Quantum Design) at temperatures between 1.8 and 300 K in magnetic fields up to 7 Tesla.The heat capacity of a small powder sample (≈1.5 mg) was measured with a PPMS system equipped with a 3 He cryostat employing the relaxation method.
The temperature dependence of the dielectric susceptibility was measured with an Andeen-Hagerling, Inc. AH 2700 ultraprecision capacitance bridge on an in situ pressed pellet of 3 mm diameter and a thickness of ∼0.5 mm and in external magnetic fields up to 3 Tesla.

Crystal structure
The new tellurium oxo-halide compound Cr 3 Te 5 O 13 Cl 3 crystallizes in the non-centrosymmetric orthorhombic space group P2 1 2 1 2 1 .The crystal structure can be regarded as layered with the layers parallel to (110) and only weak Te-Cl interactions connect the layers, see Fig.

Magnetic, thermal and dielectric properties
The magnetic susceptibilities reveal an antiferromagnetic phase transition at ∼34 K evidenced by a sharp spike which is very sensitive to the magnitude of the external magnetic field.Above 0.3 T the spike is smeared out and a broad anomaly reminiscent of ferromagnetic order remains, see Fig. 3. Isothermal magnetization measurement also reveals a critical field of about 0.25 T above which the magnetization versus field changes from linear to a Brillouin-like saturation behaviour, see Fig. 4. The high-temperature inverse susceptibilities follow Curie-Weiss behaviour (eqn (1)) with a Curie constant C corresponding to an effective magnetic moment of 3.9μ B per Cr 3+ ion, in good agreement with an S = 3/2 spin-only effective moment typically expected for Cr 3+ ions in an octahedral environment.The Curie-Weiss temperature, Θ CW , is fitted to −230 K indicating predominant antiferromagnetic spin-exchange interaction.The frustration ratio (eqn (2)) amounts to ∼6.8 indicative of sizable competing antiferromagnetic spin-exchange interaction The heat capacity reveals a sluggish weak magnetic anomaly consistent with long-range antiferromagnetic ordering at 34 K, see Fig. 5.The entropy contained in the magnetic anomaly by integration of the quantity C P /T is found to be ∼1 J mol −1 K −1 , which corresponds to only ∼10% of Rln4, expected for an S = 3/2 system; the remaining 90% apparently must have been already removed in antiferromagnetic short-range ordering processes above T C consistent with the presence of onedimensional character of the chains of edge sharing [CrO 6 ] octahedra along [100] supporting low-dimensional magnetic behaviour.
Magnetoelectric coupling may be expected to be very small since Cr 3+ with its 3d 3 electronic configuration, in a first approximation constitutes a spin-only system with small

Dalton Transactions Paper
orbital contributions to its magnetic moment and hence small coupling to lattice degrees of freedom.Nevertheless, magnetic ordering below 34 K induces an anomaly in the dielectric properties.Fig. 6 displays the temperature dependence of the dielectric constant of a polycrystalline sample of Cr 3 Te 5 O 13 Cl 3 .ε (6 kHz) amounts to ∼7.9 and decreases by about 1% on cooling from 50 K to liquid helium temperatures.Below 34 K ε(T) exhibits a magnetoelectric anomaly which is very sensitive to small external magnetic fields.In zero external field ε(T) shows a drop which is washed out by a field of 0.05 T. At 0.1 T the anomaly is inverted and ε(T) exhibits an increase and a ridge below 34 K.In fields above 1 T the magnetoelectric anomaly is completely smeared out.

Conclusions
The new tellurium oxo chloride Cr 3 Te 5 O 13 Cl 3 was synthesized via chemical reactions in evacuated and sealed silica tubes.The compound crystallizes in the non-centrosymmetric space group P2 1 2 1 2 1 .The crystal structure is built from layers The transition is shown to be very sensitive to the strength of the magnetic field applied.Above a critical field of ∼0.3 T the magnetization versus field changes from linear to a Brillouinlike saturation behaviour.The dielectric constant ε (6 kHz) amounts to ∼7.9 and decreases by about 1% on cooling from 50 K to liquid helium temperatures.Below 34 K ε(T) exhibits a magnetoelectric anomaly which is very sensitive to small external magnetic fields.

Table 1
Crystal data and structure refinement parameters for Cr 3 Te 5 O 13 Cl 3 Empirical formula Cr 3 Te 5 O 13 Cl 3 Formula weight (g mol −1 ) -fit on F 2 0.992 Final R indices a R 1 = 0.0282 [I > 2σ(I)] wR 2 = 0.0504 R indices (all data) R 1 = 0.0346 wR 2 = 0.0517 Flack parameter 0.01(3) Largest diff.peak and hole (e Å −3 ) 1.462 and −2.032 a R 1 = ∑||F o | − |F c ||/∑|F o |; wR 2 = {∑[w(F o 2 − F c with at least 4% of the valence of the cation shall be considered as coordinated 22 and this implies Te-O distances <2.66 Å and Te-Cl distances <3.05 Å, see Fig. 2. Including all such Te-O and Te-Cl distances will result in the following coordination polyhedra: [Te1O 3 ], [Te2O 3 Cl], [Te3O 4 Cl], [Te4O 4 Cl], and [Te5O 3 Cl].If all these O and Cl atoms at the verge of the primary coordination sphere are considered as bonded then only the [Te1O 3 ] polyhedra will be isolated and the other will constitute a [Te 17 O 42 Cl 12 ] polymer chain that extends along [001].The Cl2 and Cl3 atoms thus act mainly as counter ions as indicated by their low bond valence sum values (0.46 and 0.36 respectively) and they hold together the layered structure by weak interactions with tellurium atoms of neighbouring layers; this interaction is not symmetric and the chlorine atom is always closer to one of the tellurium atoms it interacts with, see Fig. 1.

Fig. 2
Fig. 2 Asymmetric unit with selected symmetry equivalents.Long Te-O and Te-Cl interactions are indicated by dashed lines.The [CrO 6 ] octahedra form chains along [100] via edge sharing.

Fig. 3
Fig. 3 Magnetic susceptibility of Cr 3 Te 5 O 13 Cl 3 measured at different magnetic fields.The antiferromagnetic ordering at 34 K is suppressed by magnetic fields above ∼0.3T. The inset displays the inverse susceptibility measured in a field of 0.3 T, the slope of the straight solid line corresponds to an effective magnetic moment of 3.9μ B per Cr 3+ ion.The straight line intersects the temperature axis at about −230 K indicating predominantly an antiferromagnetic spin-exchange interaction.
stacking parallel to the (110) plane, which are held together by weak Te-Cl interactions.The chromium atoms form CrO 6 octahedra, which are arranged in chains via edge sharing along [110].The tellurium atoms form [TeO 3 ] trigonal pyramids because of the stereochemically active lone pair.Magnetic measurements show an antiferromagnetic ordering at ∼34 K.

Fig. 5
Fig. 5 Temperature dependence of the heat capacity, C P (T), for Cr 3 Te 5 O 13 Cl 3 .The inset displays the quantity C P /T emphasizing the magnetic phase transition near 34 K (arrow).

Fig. 6
Fig. 6 Temperature dependence of the dielectric constant, ε, of a polycrystalline sample of Cr 3 Te 5 O 13 Cl 3 .