Hydrothermal synthesis of single crystal CoAs2O4 and NiAs2O4 compounds and their magnetic properties

The crystal structures of the hydrothermally synthesised trippkeite-related materials, CoAs2O4 and NiAs2O4 were investigated by means of single-crystal X-ray diffraction, Raman and infrared spectroscopy. The obtained compounds crystallise in the tetragonal crystal system (P42/mbc), with unit cell parameters at 293 K of a = 8.34530(10)/8.2277(12) A, c = 5.62010(10)/5.6120(11) A, V = 391.406(12)/379.90(13) A3, Z = 4, for CoAs2O4 and NiAs2O4 respectively. Magnetic measurements show that the resulting single crystal of NiAs2O4 exhibits an antiferromagnetic transition at TN = 53 K in a high magnetic field of 10 kOe, as already reported in the literature. The single crystal of CoAs2O4 reveals an interplay between ferromagnetic and a canted antiferromagnetic interactions resulting in a canted antiferromagnetic state which occurs at 105 K – the highest critical temperature among all similar structures.


Introduction
In order to recognise the role of arsenic in the environment, one has to investigate structures and stabilities of naturally occurring arsenic compounds. Besides, a study of mineral-related synthetic phases should be useful, because they can appear as a consequence of human activities. The susceptibility of As(III) to oxidation to As(V) in oxide environments affords high thermal stability only to ternary X-As(V)-O oxides (X ¼ Mg and divalent 3d transition metal) with arsenic in its higher oxidation state. However, under strict synthetic conditions, X-As(III)-O oxides can be formed. For that reason, anhydrous arsenates of cobalt and nickel are wellestablished but arsenites of cobalt and nickel are less known. Besides Co 2 As 2 O 5 , 1 CoAs 2 O 4 represents the second compound in the Co(II)-As(III)-O system and the NiAs 2 O 4 is the rst structurally determined compound in the Ni(II)-As(III)-O system. CoAs 2 O 4 and NiAs 2 O 4 are isostructural to the M 2+ X 2 3+ O 4 materials and minerals, [2][3][4][5][6][7][8][9] with the exception of ZnAs 2 O 4 . 3 These compounds crystallise tetragonal, adopting space group P4 2 /mbc, and contain chains of edge-linked MO 6 4 have been shown to display antiferromagnetic ordering with Néel temperatures in the range 40-60 K and a transition of the magnetic modal ordering from a predominant A mode to a C mode (vide infra) on crossing the rst row transition metals. 5,7,8,10 The edge sharing nature of the octahedra and the superexchange interactions between the M 2+ transition metals in the chains and between the chains are responsible for the magnetic properties of these group compounds. However, no crystallographic and magnetic structures have been reported for the M 2+ As 2 O 4 (M 2+ ¼ Co, Ni) compounds. Molecular susceptibility of NiAs 2 O 4 has been measured and the values of the Néel temperature and the asymptotic Curie temperature are given. 11 CoAs 2 O 4 and NiAs 2 O 4 were synthesised during an on-going research on natural and synthetic arsenic oxo-salts, with a focus on their structural and spectroscopic classication. The present article reports the hydrothermal synthesis of two new arsenites, CoAs 2 O 4 and NiAs 2 O 4 . The results of the determination of their crystal structures based on single-crystal X-ray diffraction data are given and the relationship to the known M 2+ X 2 O 4 compounds is discussed. To obtain further information on anion groups, Raman and infrared spectra were acquired. Due to the presence of transition metal M 2+ cations, non-diamagnetic ground state of as grown crystals is expected and investigated using SQUID measurements. Continuous investigations on the crystal chemistry of the arsenic oxo-salts are performed because arsenic is at the top of the priority of the most hazardous substances, but less is known about its crystal structures.

Preparation of CoAs 2 O 4 .
During an on-going research on synthetic mineral-like arsenites in the M1-M2-As(III)-(H)-(Cl) system (M1 ¼ Na + , K + , Sr 2+ , Ba 2+ ; M2 ¼ Mg 2+ , Mn 2+,3+ , Fe 2+,3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ), with a focus on their structural and spectroscopic classication, single crystals of CoAs 2 O 4 were obtained. The crystals of CoAs 2 O 4 were grown under hydrothermal conditions in Teon-lined stainless steel autoclaves from a mixture of KCl, Co(OH) 2 , As 2 O 3 and distilled H 2 O in molar ratio 1 : 1 : 1. The initial pH value of the mixture was 6. The stainless-steel autoclave was then closed and the crystallisation was carried out by placing the autoclave in oven under air atmosphere and heating the mixture under autogenous pressure from room temperature. A heating regime with three steps was chosen: the autoclaves were heated from 20 C to 200 C (four hours), held at 200 C for 199 h, and nally cooled to room temperature within 98 h. The pH value of supernatant solution was 6. The obtained products were washed with distilled water, ltered and dried in the air at room temperature. CoAs 2 O 4 crystallised as pink transparent elongated prisms (yield 60%) (Fig. 1a)  The initial pH value of the mixture was kept at around 2.5 in order to avoid the oxidation of As 3+ to As 5+ . A heating regime with three steps was chosen: the autoclaves were heated from 20 C to 220 C (four hours), held at 220 C for 56 h, and slowly cooled to room temperature within 320 h. The pH value of supernatant solution was 4. The products obtained were washed with distilled water, ltered and dried in the air at room temperature. NiAs 2 O 4 crystallised as green transparent elongated prisms (yield ca. 95%) (Fig. 1b). The maximal length of crystals was about 1.7 mm.

Single-crystal X-ray diffraction measurements
Several single crystals of the two title compounds were studied on a Bruker AXS Kappa APEX II CCD diffractometer, equipped with a monocapillary optics collimator and graphitemonochromatised MoKa radiation (l ¼ 0.71073Å). Singlecrystal X-ray diffraction data were collected at room temperature, integrated and corrected for Lorentz and polarization factors and absorption correction by evaluation of partial multiscans (see Table 1 for details). The intensity data were processed with the Bruker-Nonius programme suite SAINT-Plus 13 and corrected for Lorentz, polarization, background and absorption effects. Their crystal structures were rened with SHELXL-97 (ref. 14) and WinGX 15 starting from the atomic coordinates given for isotypic CuAs 2 O 4 . 2 Relevant information on crystal data, data collection, and renements are compiled in Table 1. For nal positional and displacement parameters see CIF les. † Selected bond lengths and angles for both arsenites are presented in Table 2.

Vibrational spectroscopy measurements
The single-crystal Raman spectra and Raman spectra of the bulk material of CoAs 2 O 4 and NiAs 2 O 4 were collected using a Horiba LabRam HR Evolution system equipped with a Si-based, Peltiercooled CCD detector in the spectral range from 4000 to 60 cm À1 . The 633 nm excitation line of a He-Ne laser was focused with a 100Â objective on the randomly oriented single crystal. The sample spectra were acquired with a nominal exposure time of 5 s and 10 s, for Co-and Ni-arsenite, respectively (confocal mode, Olympus 1800 lines per mm, 1.5 mm lateral resolution, approximately 3 mm depth resolution).
Fourier transform infrared (FTIR) absorption spectra of the title compounds were recorded using a Bruker Tensor 27 FTIR spectrophotometer, equipped with mid-IR Globar light source and KBr beam splitter, attached to a Hyperion 2000 FTIR microscope with liquid nitrogen-cooled mid-IR, broad-band MCT detector. A total of 128 scans were accumulated between 4000 and 370 cm À1 using the samples prepared as KBr pellets (KBr : MAs 2 O 4 ¼ 200 : 1).

Magnetic measurements
The magnetisation was measured with a QUANTUM DESIGN MPMS-XL-5 SQUID magnetometer. Zero-eld-cooled (ZFC) and eld-cooled (FC) runs were performed between room temperature and 2 K in a static magnetic eld of 10 kOe, 1 kOe and 100 Oe. Isothermal magnetization curves were measured at several temperatures below and above the critical temperature.

Crystal structures
In the crystal structures of MAs 2 O 4 , the chains of edge-sharing MO 6 octahedra run parallel to the [001] direction, where the individual octahedra are orientated such that the apical, M-O1 bonds lie perpendicular to [001], and are directed towards the adjacent chain, which are further interconnected with the chains of corner sharing (AsO 3 ) 3À groups ( Fig. 2 and 3).

Vibrational spectra analysis
Spectroscopic data on arsenites that have been previously published are so far rather incomplete and not in good agreement with each other. However, the spectra assignments of CoAs 2 O 4 and NiAs 2 O 4 may be based on the single-crystal Raman study of synthetic trippkeite, CuAs 2 O 4 (ref. 21) and AAsO 2 (ref. 19) (A ¼ Na, K, and Rb). In the AAsO 2 compounds, where AsO 3 units are also interconnected to the chains (each unit possessing one terminal O and two bridging O atoms), the bands above 800 cm À1 are observed in each spectrum. These bands were assigned to the vibration of terminal oxygen atoms. The bands at 810 and 780 cm À1 in synthetic trippkeite are assigned to stretches of terminal O atoms and stretches of the bridging O atoms are assigned to bands at 657 and 496 cm À1 . Therefore the distinct frequency ranges in CoAs 2 O 4 and NiAs 2 O 4 may be assigned as follows: The Raman and infrared (IR) spectra of CoAs 2 O 4 and NiAs 2 O 4 are presented at the Fig. 4-6.
The Raman spectra of both title compounds were obtained aligning the laser beam parallel and normal to the longest axis of the single-crystal. The strong bands at 776 and 774 cm À1 (778 cm À1 in IR-spectrum and 783 cm À1 in the Raman spectrum of the bulk) in NiA 2 O 4 and 787 and 780 cm À1 (767 cm À1 in IR-spectrum and 769 cm À1 in the Raman spectrum of the bulk) in CoAs 2 O 4 in parallel (p) and normal (n) orientation to the laser beam, respectively may be assigned to the symmetric As-O terminal stretches, and the weak bands between 700 and 450 cm À1 in both orientations and the bulk Raman spectra are assigned to the As-O bridging stretches (strong bands at 533 and 494 cm À1 and 540, 516, 486 cm À1 in IR-spectrum of NiAs 2 O 4 and CoAs 2 O 4 , respectively). In the p orientation of the Raman spectra, bands of very low intensity were observed at 960 and 958 cm À1 (around 960 cm À1 in IR spectra) as well as a shoulder to the bands around 780 cm À1 at 831 and 822 cm À1 in NiAs 2 O 4 and CoAs 2 O 4 , respectively. These bands are attributed also to  the symmetric As-O terminal stretches. The very weak bands being seen only in p orientation at 747 and at the bulk spectrum (strong band at 753 cm À1 ) and 646 cm À1 in NiAs 2 O 4 and 638 cm À1 (strong band at 739 cm À1 ) in CoAs 2 O 4 , respectively, may be attributed to the antisymmetric stretches. It is suggested that the (AsO 3 ) 3À group is the only tetrahedral oxyanion of the main group elements in which n s > n as . 22 The same is suggested for (As 2 O 4 ) 2À group by Bencivenni and Gingerich. 23 These authors noted that it was unusual for the vibrational spectroscopy of oxy-anions. Further strong bands in the spectra of both arsenites are around 350 cm À1 (357 and 352

Magnetic properties
The magnetic properties of the NiAs 2 O 4 and CoAs 2 O 4 single crystals were investigated, since the presence of transition metals in chemical composition with unpaired d electrons indicates the non-diamagnetic ground state. All single crystals used for the magnetic measurements were studied by singlecrystal X-ray diffraction techniques. The measurements showed the same primitive tetragonal unit cell, without additional non-indexed reections. All single crystals have been probed in the constant magnetic eld oriented with their c-axis parallel to the magnetic eld (H k c), while the crystals of CoAs 2 O 4 have been measured also in H t c orientation. The temperature dependence of the inverse molar magnetic susceptibility (measured at 10 kOe and displayed in Fig. 7) has a typical paramagnetic shape between room temperature and approximately 140 K, and can be described in the high temperature range (140-300 K) with the Curie-Weiss law: where c 0 is the temperature-independent part of c, i.e. diamagnetic contribution, C is the Curie constant, and q is the Curie-Weiss temperature. From the slope of c À1 vs. T graph we obtain p eff ¼ 3.3 m B and Curie-Weiss temperature q of approximately À40 K for NiAs 2 O 4 as reported in Witteveen, 11 while p eff ¼ 5.6 m B and q z 75 K for CoAs 2 O 4 sample. The paramagnetic effective moments p eff calculated from the Curie constant are in a close agreement with the literature data for Ni 2+ cation (3.2 m B ) and high spin Co 2+ cation with large orbital contribution (6.5 m B ). 25 Below 140 K the susceptibility of CoAs 2 O 4 sample starts to deviate from paramagnetic (linear c À1 vs. T) regime, while for NiAs 2 O 4 the same occurs below 70 K. We also performed an equivalent analysis of inverse molar magnetic susceptibility measured under external eld of 1 kOe, and obtained results are practically identical as for previously described measurement (Fig. S1 -ESI †). Magnetic susceptibility measurements of NiAs 2 O 4 under ZFC and FC conditions (Fig. 8a) show some unexpected results. At T N ¼ 53 K in high magnetic eld of 10 kOe, the susceptibility   shows behavior consistent with a transition to the antiferromagnetic ground state without splitting between ZFC and FC curves as already reported. 24 When the applied DC eld is ten times lower, at the same temperature ZFC and FC curves start to diverge. Such divergence is highly enhanced when applied eld was 100 Oe only. This rst observed strong dependence of the transition at 55 K on magnetic eld might be a consequence of two different superexchange interactions between Ni 2+ ions: a positive and weaker one intrachain J 1 between magnetic moments in the chain, and a negative and stronger interchain interaction J 1 as proposed by Witteveen. 11 The unexpected behaviour of ZFC-FC curves is detected at the temperature around 20 K, where the transition to the ferromagnetic state has been clearly observed. Finally, below 15 K ZFC-FC curves show a strong divergence.
This second magnetic transition inuences also the magnetization curves as only at 2 K where a small hysteresis with coercivity H c ¼ 1.6 kOe and remanent magnetization M r ¼ 7.5 Â 10 À3 m B /Ni atom can be observed in M-H graph displayed in Fig. 8b. M(H) curve measured at 50 K and 70 K is rather linear.
The phase transition from paramagnetic to magnetic ordered phase in CoAs 2 O 4 occurs at higher temperature as in NiAs 2 O 4 . The positive Curie-Weiss temperature w ¼ 75 K obtained from c À1 vs. T plot suggests a ferromagnetic interaction between cobalt ions. Indeed, with decreasing temperature, the susceptibility measured in H ¼ 100 Oe suddenly sharply increases at T c ¼ 105.5 K. The critical temperature T c was dened from the t M f (1 À T/T c ) b as shown in inset in Fig. 9a. The obtained critical exponent b ¼ 0.34 agrees with the theoretically calculated value for 3-D Ising system. 26 The susceptibility at T c and below behaves quite differently when measured in 1 kOe or 10 kOe instead of 100 Oe. It decreases below T c as it is characteristic for antiferromagnetic transitions. The magnetisation curves M vs. H measured at several temperatures (Fig. 9b) also show this dualityferromagnetic and antiferromagnetic behaviour. M(H) obtained at 2 K and 10 K exhibits an "S"-shaped curve for small magnetic elds that saturates at z0.01 m B /(Co ion) in a eld of approx. 5 kOe. In larger magnetic elds magnetisation increases linearly with the elds as expected for antiferromagnetically coupled magnetic moments.
The   compounds with the exception of CoSb 2 O 4 where it is roughly the same. This may be the reason why the transition to magnetically ordered phase is shied to a higher temperature for CoAs 2 O 4 . Practically the same critical exponent as we obtained for CoAs 2 O 4 (b ¼ 0.34) was measured in MnSb 2 O 4 (ref. 5) (b ¼ 0.36) too. Having these similar structural and detected magnetic properties in mind, we propose the samea canted antiferromagnetic structure of CoAs 2 O 4 as described for MnSb 2 O 4 . 5 Such a structure is in agreement with a measured ferromagnetic response in a small magnetic eld and prevailing antiferromagnetism when measured in magnetic eld of 1 kOe or larger. In order to conrm the proposed magnetic structure neutron diffraction data are needed, for which there is not enough material at the moment.
Below 10 K a similar increase of the susceptibility in CoAs 2 O 4 can be observed as already described for NiAs 2 O 4 below 20 K. At the moment we have no reliable explanation for these two increases of susceptibilities. Similar anomaly in the ZFC/FC susceptibility below about 20 K has been already detected for NiO nanoparticles and bulk materials. [27][28][29] The anomaly was contributed to surface spin magnetism due to Ni 2+ magnetic moments that are not coordinated in the same way as expected for the titled compound. We tentatively ascribe the measured anomalies at 20 K and 10 K in NiAs 2 O 4 and CoAs 2 O 4 , respectively, to the surface spins. In order to test this hypothesis much larger single crystals as we used in our research are needed.
The single-crystalline nature of investigated compounds points a further magnetic research into the possible magnetic anisotropy detection. In high magnetic eld (1 T) the parallel and perpendicular susceptibilities, as shown in the inset of Fig. 10, are as expected for typical two-dimensional (layered) antiferromagnetic system as already described for BaNi 2 (PO 4 ) 2 and Rb 2 Co 0.7 Mg 0.3 F 4 . 26 While in small magnetic eld of 100 Oe susceptibilities in both orientations of the sample increases below T N . The increase is even larger for perpendicular orientation, in agreement with our hypothesis of canted magnetic moments from c-direction.

Conclusions
CoAs 2 O 4 and NiAs 2 O 4 were synthesised using low-temperature hydrothermal method, which resulted in beautiful, pure and homogeneous single crystals. In this manner, among isotypic M 2+ X 2 3+ O 4 compounds, the measuring of the magnetic properties using single-crystals was possible for the rst time. Singlecrystals also made us possible to extend our study of magnetic and vibrational properties to anisotropy measurements. MX 2 O 4 compounds could be obtained by different synthesis routes (ux method, solid-state reaction, high-temperature hydrothermal method), but with the exception of CuAs 2 O 4 , lowtemperature hydrothermal method was used for the crystallization of suitable M 2+ X 2 3+ O 4 material for the rst time.
The structurally characterised single crystal of NiAs 2 O 4 , exhibit magnetic properties in accordance with the reported data. The magnetic susceptibility of NiAs 2 O 4 at T N ¼ 53 K in high magnetic eld of 10 kOe shows behaviour consistent with a transition to the antiferromagnetic ground state. However, at 20 K another transition to the ferromagnetic state has been clearly observed, which might be attributed to the uncompensated surface spins due to Ni 2+ magnetic moments that are not coordinated in the same way as expected for the title compound. The SQUID measurement of the single crystal of CoAs 2 O 4 reveals some subtile interplay between AFM and FM interactions in the system as evidenced as FM-like transition at 105.5 K in small magnetic eld and AFM-like transition in 10 kOe and above. The transition at 105.5 K is, according to our knowledge, the highest critical temperature among all similar structures.