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

Abstract

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) Å, c = 5.62010(10)/5.6120(11) Å, V = 391.406(12)/379.90(13) ų, Z = 4, for CoAs₂O₄ and NiAs₂O₄ respectively. Magnetic measurements show that the resulting single crystal of NiAs₂O₄ exhibits an antiferromagnetic transition at T_N = 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.

---

1. 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 = 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 well-established but arsenites of cobalt and nickel are less known. Besides Co₂As₂O₅,¹ CoAs₂O₄ represents the second compound in the Co(II)–As(III)–O system and the NiAs₂O₄ is the first structurally determined compound in the Ni(II)–As(III)–O system.

CoAs₂O₄ and NiAs₂O₄ are isostructural to the M²⁺X₂³⁺O₄ materials and minerals,^2–9 with the exception of ZnAs₂O₄.³ These compounds crystallise tetragonal, adopting space group P4₂/mbc, and contain chains of edge-linked MO₆ octahedra running along [001]; the chains are connected via trigonal pyramidal XO₃ units.

All MSb₂O₄ phases, with the exception of MgSb₂O₄,⁴ 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 first row transition metals.^5,7,8,10 The edge sharing nature of the octahedra and the superexchange interactions between the M²⁺ 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 M²⁺As₂O₄ (M²⁺ = Co, Ni) compounds. Molecular susceptibility of NiAs₂O₄ has been measured and the values of the Néel temperature and the asymptotic Curie temperature are given.¹¹

CoAs₂O₄ and NiAs₂O₄ were synthesised during an on-going research on natural and synthetic arsenic oxo-salts, with a focus on their structural and spectroscopic classification. The present article reports the hydrothermal synthesis of two new arsenites, CoAs₂O₄ and NiAs₂O₄. 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 M²⁺X₂O₄ compounds is discussed. To obtain further information on anion groups, Raman and infrared spectra were acquired. Due to the presence of transition metal M²⁺ 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.

2. Experimental

2.1 Materials

The materials used were: KCl (Loba Chemie, 13508), Co(OH)₂ (Sigma-Aldrich, 342440, tech. 95%), Ni(NO₃)₂ (Sigma-Aldrich 244074-500G), As₂O₃ (Alfa Aesar 33289, ACS 99.95–100.05%).

2.1.1 Preparation of CoAs₂O₄. During an on-going research on synthetic mineral-like arsenites in the M1–M2–As(III)–(H)–(Cl) system (M1 = Na⁺, K⁺, Sr²⁺, Ba²⁺; M2 = Mg²⁺, Mn^2+,3+, Fe^2+,3+, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺), with a focus on their structural and spectroscopic classification, single crystals of CoAs₂O₄ were obtained. The crystals of CoAs₂O₄ were grown under hydrothermal conditions in Teflon-lined stainless steel autoclaves from a mixture of KCl, Co(OH)₂, As₂O₃ and distilled H₂O 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 finally cooled to room temperature within 98 h. The pH value of supernatant solution was 6. The obtained products were washed with distilled water, filtered and dried in the air at room temperature. CoAs₂O₄ crystallised as pink transparent elongated prisms (yield 60%) (Fig. 1a) together with transparent crystals of KAs₄O₆Cl¹² (yield 40%). The maximal length of crystals was about 2 mm.

Fig. 1 Single crystals of (a) CoAs₂O₄ and (b) NiAs₂O₄.

2.1.2 Preparation of NiAs₂O₄. After successful synthesis of the single crystals of CoAs₂O₄, systematic synthesis of the single crystals of M²⁺As₂O₄ (M = Mn, Fe, Co, Ni) was performed. However, only attempts to get single crystals of CoAs₂O₄ and NiAs₂O₄ were fruitful. The crystals of NiAs₂O₄ were grown from a mixture of Ni(NO₃)₂, As₂O₃ and distilled H₂O in molar ratio 1 : 1. The initial pH value of the mixture was kept at around 2.5 in order to avoid the oxidation of As³⁺ to As⁵⁺. 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, filtered and dried in the air at room temperature. NiAs₂O₄ crystallised as green transparent elongated prisms (yield ca. 95%) (Fig. 1b). The maximal length of crystals was about 1.7 mm.

2.2 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 graphite-monochromatised MoKα radiation (λ = 0.71073 Å). Single-crystal 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-Plus¹³ and corrected for Lorentz, polarization, background and absorption effects. Their crystal structures were refined with SHELXL-97 (ref. 14) and WinGX¹⁵ starting from the atomic coordinates given for isotypic CuAs₂O₄.²

Table 1 Crystal data, data collection and refinement details for CoAs₂O₄ and NiAs₂O₄

Crystal data
a w = 1/[σ^2(Fo^2) + (0197P)^2 + 0.1338P] for CoAs2O4, w = 1/[σ^2(Fo^2) + (0.0129P)^2 + 0.2839P] for NiAs2O4.
Chemical formula	CoAs2O4	NiAs2O4
Temperature (K)	293(2)	293(2)
Crystal form, colour	Prismatic, pink	Prismatic, green
Formula weight, Mr (g mol^−1)	272.77	272.55
System, space group (no.)	Tetragonal, P42/mbc (135)
a (Å)	8.34530(10)	8.2277(12)
c (Å)	5.62010(10)	5.6120(11)
V (Å^3)	391.406(12)	379.90(13)
Z	4	4
F(000)	500	504
Calculated density, Dx (g cm^−3)	4.629	4.765
Absorption coefficient, μ (mm^−1)	21.032	22.258
Transmission factors, Tmin/Tmax	0.124/0.678	0.135/0.664
Crystal size (mm)	0.02 × 0.02 × 0.17	0.02 × 0.02 × 0.15
Reflections collected/unique	11 599/838	7925/595
Observed reflections [I > 2 σ(I)]	719	532
R int	0.0418	0.0382
Range for data collection, θ (°)	3.452–44.387	3.502–39.118
Range of Miller indices	−15 ≤ h ≤ 16; −16 ≤ k ≤ 16; −10 ≤ l ≤ 9	−14 ≤ h ≤ 14; −8 ≤ k ≤ 12; −9 ≤ l ≤ 9
Extinction coefficient, k^a	0.0159(8)	0.0056(5)
Refined parameters	22	22
R-indices [I > 2 σ(I)]^a	R 1 = 0.0229	R 1 = 0.0161
	wR2 = 0.0440	wR2 = 0.0363
R-indices (all data)^a	R 1 = 0.0306	R 1 = 0.0371
	wR2 = 0.0455	wR2 = 0.059
Goodness-of-fit, S	1.079	1.09
(Δρ)max, (Δρ)min (e Å^−3)	1.088, −1.022	0.490, −0.656

---

Relevant information on crystal data, data collection, and refinements are compiled in Table 1. For final positional and displacement parameters see CIF files.† Selected bond lengths and angles for both arsenites are presented in Table 2.

Table 2 Selected bond distances (Å) and angles (°) for CoAs₂O₄ and NiAs₂O₄^a

CoAs2O4		NiAs2O4
a Symmetry codes: (i) y − 1/2, x + 1/2, −z + 1/2; (ii) −x, −y + 1, −z; (iii) −y + 1/2, −x + 1/2, z + 1/2; (iv) −x, −y + 1, z; (v) −x + 1/2, y − 1/2, −z; (vi) x, y, −z.
Co–O2	2.0305(7)	Ni–O2	2.0079(9)
–O2^i	2.0305(7)	–O2^i	2.0079(9)
–O2^ii	2.0305(7)	–O2^ii	2.0079(9)
–O2^iii	2.0305(7)	–O2^iii	2.0079(9)
–O1	2.1775(11)	–O1	2.1222(13)
–O1^iv	2.1775(11)	–O1^iv	2.1222(13)
〈Co–O〉	2.0795	〈Ni–O〉	2.046
As–O2^v	1.7292(11)	As–O2^v	1.7275(12)
–O1^vi	1.8473(5)	–O1^vi	1.8501(6)
–O1	1.8473(5)	–O1	1.8501(6)
〈As–O〉	1.8079	〈As–O〉	1.8092
Co–Co	2.81005(5)	Ni–Ni	2.8060(5)
∠O1b–As–O2t	96.76(3) × 2	∠O1b–As–O2t	96.64(4) × 2
∠O1b–As–O1b	99.03(4)	∠O1b–As–O1b	98.63(4)
∠As–O1b–As	124.09(6)	∠As–O1b–As	124.13(7)

---

2.3 Vibrational spectroscopy measurements

The single-crystal Raman spectra and Raman spectra of the bulk material of CoAs₂O₄ and NiAs₂O₄ were collected using a Horiba LabRam HR Evolution system equipped with a Si-based, Peltier-cooled CCD detector in the spectral range from 4000 to 60 cm⁻¹. The 633 nm excitation line of a He–Ne laser was focused with a 100× 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 μm lateral resolution, approximately 3 μm 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⁻¹ using the samples prepared as KBr pellets (KBr : MAs₂O₄ = 200 : 1).

2.4 Magnetic measurements

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

3. Results and discussion

3.1 Crystal structures

In the crystal structures of MAs₂O₄, the chains of edge-sharing MO₆ 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 (AsO₃)³⁻ groups (Fig. 2 and 3).

Fig. 2 Part of the crystal structure of the tetragonal M²⁺X₂³⁺O₄ compounds showing the connection of the MO₆ octahedral chains via trigonal ψ-(AsO₃)³⁻ pyramids.

Fig. 3 Perspective view of the crystal structure of the tetragonal M²⁺X₂³⁺O₄ compounds.

The average Co–O and Ni–O bond lengths are 2.0795 and 2.046 Å. According to the formula Δ_oct = 1/6 Σ[(d_i − d_m)/d_m]² the bond-length distortions for the Co and Ni atoms amount to 1.11 × 10⁻³ and 6.92 × 10⁻⁴, respectively and indicate large distortions.^16,17 These results compare well with the values compiled by Wildner¹⁸ for CoO₆ octahedra in accurately determined crystal structures, who found 672 Co–O bond lengths between 1.959 and 2.517 Å. The average 〈Co–O〉 bond lengths for 112 polyhedra are in the range of 2.054 to 2.182 Å; the overall mean value is 2.1115 Å.

The MO₆ octahedra are edge-linked to chains parallel to the 4-fold axis. The shared edges O2–O2ⁱ (i = −x, −y + 1, −z) have lengths of 2.932(2) and 2.873(2) Å, for Co and NiAs₂O₄, respectively. Due to this connection, the angular distortion is large: σ_oct² = 1/11 Σ(∠_i − 90)² is 20.59 and 17.49 for the two CoO₆ and NiO₆ octahedra, respectively.^15,16 The inter-transition metal separation distances, Co–Co and Ni–Ni along [001] are equal to the c/2 and amount 2.81005(5) and 2.8060(5) Å.

Arsenic is one-sided coordinated to three oxygen atoms and its coordination figure is represented by ψ-(AsO₃)³⁻ pyramids (ψ is a stereoactive lone pair of electrons), where the As atoms lies at the vertex and the oxygen atoms are at the basis of the pyramid. ψ-(AsO₃)³⁻ pyramids share corners thus forming chains also parallel to the 4-fold axis. Oxygen atoms of the pyramid basis lie parallel to the vertex and the oxygen atoms are at the basis of the pyramid. ψ-(AsO₃)³⁻ pyramids share corners thus forming chains also parallel to the 4-fold axis. Oxygen atoms of the pyramid basis lie parallel to the (110) plane, and the As-atoms alternate left and right from that plane (Fig. 3). The As–O bond lengths within the single chain are significantly longer (1.8473(5) and 1.8501(6) Å, respectively) than the third As–O bond length (1.7292(11) and 1.7275(12) Å, respectively). As–O1_b–As angles within a single chain are 124.09(6)° and 124.13(7)° for CoAs₂O₄ and NiAs₂O₄, respectively and compare well with the As–O_b–As angles in other ‘chain’ arsenites [127.3(3)° in CuAs₂O₄,² from 123.3(5) to 125.1(3)° in XAsO₂,¹⁹ (X = Na, K, Rb), 125.1(14)° in Cs₃As₅O₉ (ref. 18) and 121.3(5) and 122.3(5) in leitite, ZnAs₂O₄.³

Bond valence sum calculations²⁰ based on the room temperature data show the models to be compatible with the bonding requirements of Co²⁺ (2.10 v.u.), Ni²⁺ (2.08 v.u.) and As³⁺ (2.88/2.88 v.u.) O1 (2.24/2.26 v.u.), O2 (1.98/1.94 v.u.) for CoAs₂O₄ and NiAs₂O₄, respectively.

3.2 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 CoAs₂O₄ and NiAs₂O₄ may be based on the single-crystal Raman study of synthetic trippkeite, CuAs₂O₄ (ref. 21) and AAsO₂(ref. 19) (A = Na, K, and Rb). In the AAsO₂ compounds, where AsO₃ units are also interconnected to the chains (each unit possessing one terminal O and two bridging O atoms), the bands above 800 cm⁻¹ are observed in each spectrum. These bands were assigned to the vibration of terminal oxygen atoms. The bands at 810 and 780 cm⁻¹ 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⁻¹. Therefore the distinct frequency ranges in CoAs₂O₄ and NiAs₂O₄ may be assigned as follows:

The Raman and infrared (IR) spectra of CoAs₂O₄ and NiAs₂O₄ are presented at the Fig. 4–6.

Fig. 4 The single-crystal Raman spectra of CoAs₂O₄ and NiAs₂O₄ presented in two different orientations. p = elongation of the crystal is parallel to the laser beam, and n = elongation of the crystal is normal to the laser beam.

Fig. 5 Raman spectra of the bulk CoAs₂O₄ and NiAs₂O₄ crystals.

Fig. 6 Infrared spectra of CoAs₂O₄ and NiAs₂O₄.

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⁻¹ (778 cm⁻¹ in IR-spectrum and 783 cm ⁻¹ in the Raman spectrum of the bulk) in NiA₂O₄ and 787 and 780 cm⁻¹ (767 cm⁻¹ in IR-spectrum and 769 cm ⁻¹ in the Raman spectrum of the bulk) in CoAs₂O₄ 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⁻¹ in both orientations and the bulk Raman spectra are assigned to the As–O_bridging stretches (strong bands at 533 and 494 cm⁻¹ and 540, 516, 486 cm⁻¹ in IR-spectrum of NiAs₂O₄ and CoAs₂O₄, respectively). In the p orientation of the Raman spectra, bands of very low intensity were observed at 960 and 958 cm⁻¹ (around 960 cm⁻¹ in IR spectra) as well as a shoulder to the bands around 780 cm⁻¹ at 831 and 822 cm⁻¹ in NiAs₂O₄ and CoAs₂O₄, 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⁻¹) and 646 cm⁻¹ in NiAs₂O₄ and 638 cm⁻¹ (strong band at 739 cm⁻¹) in CoAs₂O₄, respectively, may be attributed to the antisymmetric stretches. It is suggested that the (AsO₃)³⁻ group is the only tetrahedral oxyanion of the main group elements in which ν_s > ν_as.²² The same is suggested for (As₂O₄)²⁻ 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⁻¹ (357 and 352 cm⁻¹ in NiAs₂O₄ and 354 and 347 cm⁻¹ in CoAs₂O₄) and are assigned to the ν₄ As₂O₄²⁻ bending modes. Below 300 cm⁻¹ appear the various lattice modes of the compounds. The structure of Co- and NiAs₂O₄ consists of arrays of As–O–As–O–As chains. Two types of components are predicted based upon OAsO and AsOAs units. The bands between 300 and 200 cm⁻¹ in the both compounds are assigned to the AsOAs linkages.²⁴

3.3 Magnetic properties

The magnetic properties of the NiAs₂O₄ and CoAs₂O₄ 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 single-crystal X-ray diffraction techniques. The measurements showed the same primitive tetragonal unit cell, without additional non-indexed reflections. All single crystals have been probed in the constant magnetic field oriented with their c-axis parallel to the magnetic field (H ‖ c), while the crystals of CoAs₂O₄ have been measured also in H ⊥ 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: χ(T) = χ₀ + C/(T − θ), where χ₀ is the temperature-independent part of χ, i.e. diamagnetic contribution, C is the Curie constant, and θ is the Curie–Weiss temperature. From the slope of χ⁻¹vs. T graph we obtain p_eff = 3.3 μ_B and Curie–Weiss temperature θ of approximately −40 K for NiAs₂O₄ as reported in Witteveen,¹¹ while p_eff = 5.6 μ_B and θ ≈ 75 K for CoAs₂O₄ sample. The paramagnetic effective moments p_eff calculated from the Curie constant are in a close agreement with the literature data for Ni²⁺ cation (3.2 μ_B) and high spin Co²⁺ cation with large orbital contribution (6.5 μ_B).²⁵ Below 140 K the susceptibility of CoAs₂O₄ sample starts to deviate from paramagnetic (linear χ⁻¹vs. T) regime, while for NiAs₂O₄ the same occurs below 70 K. We also performed an equivalent analysis of inverse molar magnetic susceptibility measured under external field of 1 kOe, and obtained results are practically identical as for previously described measurement (Fig. S1 – ESI†).

Fig. 7 Temperature dependence of the inverse magnetic susceptibility for the studied NiAs₂O₄ and CoAs₂O₄ single crystals measured along crystal c-axis in a DC field H = 10 kOe. The solid (black) lines are the best fit to the Curie–Weiss law at high temperatures.

Magnetic susceptibility measurements of NiAs₂O₄ under ZFC and FC conditions (Fig. 8a) show some unexpected results. At T_N = 53 K in high magnetic field 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 field is ten times lower, at the same temperature ZFC and FC curves start to diverge. Such divergence is highly enhanced when applied field was 100 Oe only. This first observed strong dependence of the transition at 55 K on magnetic field might be a consequence of two different superexchange interactions between Ni²⁺ ions: a positive and weaker one intrachain J₁ between magnetic moments in the chain, and a negative and stronger interchain interaction J₁ 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.

Fig. 8 (a) ZFC and FC molar magnetization versus temperature curves for NiAs₂O₄ measured in the various magnetic fields aligned along crystal c-axis. (b) Magnetization versus applied field aligned along crystal c axis for NiAs₂O₄ at 2, 15, 50 and 70 K.

This second magnetic transition influences also the magnetization curves as only at 2 K where a small hysteresis with coercivity H_c = 1.6 kOe and remanent magnetization M_r = 7.5 × 10⁻³μ_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 CoAs₂O₄ occurs at higher temperature as in NiAs₂O₄. The positive Curie–Weiss temperature ϑ = 75 K obtained from χ⁻¹vs. T plot suggests a ferromagnetic interaction between cobalt ions. Indeed, with decreasing temperature, the susceptibility measured in H = 100 Oe suddenly sharply increases at T_c = 105.5 K. The critical temperature T_c was defined from the fit M ∝ (1 − T/T_c)^β as shown in inset in Fig. 9a. The obtained critical exponent β = 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 duality – ferromagnetic and antiferromagnetic behaviour. M(H) obtained at 2 K and 10 K exhibits an “S”-shaped curve for small magnetic fields that saturates at ≈0.01 μ_B/(Co ion) in a field of approx. 5 kOe. In larger magnetic fields magnetisation increases linearly with the fields as expected for antiferromagnetically coupled magnetic moments.

Fig. 9 (a) ZFC and FC molar magnetization versus temperature curves for CoAs₂O₄ measured in the various magnetic fields aligned along crystal c-axis. Full line in inset is a fit M ∝ (1 − T/T_c)^β. (b) Magnetization versus applied field aligned along crystal c axis for CoAs₂O₄ at 2 K, 10 K, and 120 K (inset).

The crystal structure of CoAs₂O₄ compound and the distribution of magnetic Co²⁺ ions in chains are similar to already reported NiAs₂O₄ (ref. 22) (T_N = 53.5 K), NiSb₂O₄ (ref. 22) (T_N = 47 K), MnSb₂O₄ (ref. 5) (T_N = 55 K), and CoSb₂O₄ (ref. 8) (T_N = 79 K). In the case of CoAs₂O₄ the magnetic ions Co²⁺ possess the largest magnetic moment among above mentioned compounds with the exception of CoSb₂O₄ where it is roughly the same. This may be the reason why the transition to magnetically ordered phase is shifted to a higher temperature for CoAs₂O₄. Practically the same critical exponent as we obtained for CoAs₂O₄ (β = 0.34) was measured in MnSb₂O₄ (ref. 5) (β = 0.36) too. Having these similar structural and detected magnetic properties in mind, we propose the same – a canted antiferromagnetic structure of CoAs₂O₄ as described for MnSb₂O₄.⁵ Such a structure is in agreement with a measured ferromagnetic response in a small magnetic field and prevailing antiferromagnetism when measured in magnetic field of 1 kOe or larger. In order to confirm 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 CoAs₂O₄ can be observed as already described for NiAs₂O₄ 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.^27–29

The anomaly was contributed to surface spin magnetism due to Ni²⁺ 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 NiAs₂O₄ and CoAs₂O₄, 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 field (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 BaNi₂(PO₄)₂ and Rb₂Co_0.7Mg_0.3F₄.²⁶ While in small magnetic field 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.

Fig. 10 FC molar magnetization versus temperature curves for CoAs₂O₄ measured in 100 Oe in both alignment configurations (along crystal c-axis, and perpendicular to it). In the inset, it was shown a corresponding high-field measurement (10 kOe).

4. Conclusions

CoAs₂O₄ and NiAs₂O₄ were synthesised using low-temperature hydrothermal method, which resulted in beautiful, pure and homogeneous single crystals. In this manner, among isotypic M²⁺X₂³⁺O₄ compounds, the measuring of the magnetic properties using single-crystals was possible for the first time. Single-crystals also made us possible to extend our study of magnetic and vibrational properties to anisotropy measurements. MX₂O₄ compounds could be obtained by different synthesis routes (flux method, solid-state reaction, high-temperature hydrothermal method), but with the exception of CuAs₂O₄, low-temperature hydrothermal method was used for the crystallization of suitable M²⁺X₂³⁺O₄ material for the first time.

The structurally characterised single crystal of NiAs₂O₄, exhibit magnetic properties in accordance with the reported data. The magnetic susceptibility of NiAs₂O₄ at T_N = 53 K in high magnetic field 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 Ni²⁺ magnetic moments that are not coordinated in the same way as expected for the title compound. The SQUID measurement of the single crystal of CoAs₂O₄ 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 field 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.

Acknowledgements

The authors gratefully acknowledge financial support from the Austrian Science Foundation (FWF) (Grant no. V203-N19), Austrian Student Exchange Service (ÖAD) and Austrian Ministry of Science (BM.W_f) (Bilateral cooperation project with Croatia 2014–15, Grant no. HR05/2014). The authors are thankful to Andreas Artač, M.Sc and Prof. Dr Eugen Libowitzky for assisting during spectroscopic analysis.

References

1. S. Lösel and H. Hillebrecht, Z. Anorg. Allg. Chem., 2008, 634, 2299–2302.
2. F. Pertlik, TMPM, Tschermaks Mineral. Petrogr. Mitt., 1975, 22, 211–217.
3. S. Ghose, P. K. S. Gupta and E. O. Schlemper, Am. Mineral., 1987, 72, 629–632.
4. C. Giroux-Maraine and G. Perez, Rev. Chim. Miner., 1975, 12, 427–432.
5. H. Fjellvåg and A. Kjekshus, Acta Chem. Scand., Ser. A, 1985, 39, 389–395.
6. R. Fischer and F. Pertlik, TMPM, Tschermaks Mineral. Petrogr. Mitt., 1975, 22, 236–241.
7. J. R. Gavarri, R. Chater and J. Ziółkowski, J. Solid State Chem., 1988, 73, 305–316.
8. B. P. De Laune and C. Greaves, J. Solid State Chem., 2012, 187, 225–230.
9. E. G. Puebla, E. G. Ríos, A. Monge and I. Rasines, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1982, 38, 2020–2022.
10. R. Chater, J. R. Gavarri and A. W. Hewat, J. Solid State Chem., 1985, 60, 78–86.
11. H. T. Witteveen, Solid State Commun., 1971, 9, 1313–1315.
12. F. Pertlik, Monatsh. Chem., 1988, 119, 4581.
13. SAINT, Bruker AXS Inc., 5465 East Cheryl Parkway, Madison, WI, USA, 53711–5373, 2000.
14. G. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112–122.
15. L. J. Farrugia, J. Appl. Crystallogr., 2012, 45, 849–854.
16. K. Robinson, G. V. Gibbs and P. H. Ribbe, Science, 1971, 172, 567–570.
17. M. E. Fleet, Mineral. Mag., 1976, 40, 531–533.
18. M. Wildner, Z. Kristallogr., 1992, 202, 51–70.
19. F. Emmerling and C. Röhr, Z. Naturforsch., B: J. Chem. Sci., 2003, 58, 620–626.
20. N. E. Brese and M. O'Keeffe, Acta Crystallogr., Sect. B: Struct. Sci., 1988, 47, 192–197.
21. S. Bahfenne, L. Rintoul and R. L. Frost, Am. Mineral., 2011, 96, 888–894.
22. A. G. Nord, P. Kierkegaard, T. Stefanidis and J. Baran, Chem. Commun., 1988, 1–40.
23. L. Bencivenni and K. A. Gingerich, J. Mol. Struct., 1983, 99, 23–29.
24. R. L. Frost and S. Baffenne, J. Raman Spectrosc., 2010, 41, 325–328.
25. N. W. Ascroft and N. D. Mermin, Solid State Physics, Saunders College Publishing, USA, 1976, pp. 657–658.
26. L. J. De Jongh, Magnetic properties of layered transition metal compounds, Kluwer Academic Publishers, Netherland, 1990.
27. F. Bodker, M. F. Hansen, C. Bender Koch and S. J. Morup, J. Magn. Magn. Mater., 2000, 221, 32–36.
28. M. Jagodič, Z. Jagličić, A. Jelen, J.-B. Lee, Y.-M. Kim, H. J. Kim and J. Dolinšek, J. Phys.: Condens. Matter, 2009, 21, 215302.
29. H. Shim, P. Dutta, M. S. Seehra and J. Bonevich, Solid State Commun., 2008, 145, 192–196.

---

Footnote

† Electronic supplementary information (ESI) available: CIF files of the structures. See DOI: 10.1039/c4ra16122j

---