Crystal Growth of a New 8H Perovskite Sr8Os6.3O24 Exhibiting High TC Ferromagnetism

Single crystals of a new twinned hexagonal perovskite compound Sr8Os6.3O24 have been synthesized, and structural and magnetic properties have been determined. The compound crystallizes in a hexagonal cell with lattice parameters a = 9.6988(3) {\AA} and c = 18.1657(5) {\AA}. The structure is an eight-layered hexagonal B-site deficient perovskite with the layer sequence (ccch)2 and represents the first example of a hexagonal structure among 5d oxides adopting a twin option. The sample shows spontaneous ferromagnetic magnetization below 430 K with a small saturation moment of 0.11 {\mu}B/Os ion. This is the highest Curie temperature (TC) reported for any bulk perovskite containing only 5d ions at the B site.


INTRODUCTION
Perovskites represent one of most robust and perhaps the most structurally variable classes of oxides. 1,2 The basic structure type ABO3 allows for replacements at A and B sites in different ratios and for a wide variety of stacking sequences of the AO3 dense packed layers, which results in a vast combinatorial diversity of individual configurations. As one of the consequences, this family of structures offers extensive opportunities for tuning the physical properties. Among the factors of influence, the Goldschmidt tolerance factor may serve as a quite reliable guide in anticipating distortions and stacking variants of any targeted perovskite structure. 3 The two ideal types of perovskites feature a cubic (c) or hexagonal (h) stacking sequence of the AO3 layers, and virtually any combination of c and h sequences is possible. 2 In particular among the hexagonal perovskites, different combinatorial stacking of c and h layers leads to two different structure options: shift and twin, where the shift structures have a sequence of (cnhh; n = number of layers) layers and twin structures are built up of (cnh) layers. 2 The degrees of freedom mentioned give rise to interesting structure related properties such as microwave dielectrics (some recent examples are La5MxTi4-xO15, Ba8CoNb6O24, Ba10Mg0.25Ta7.9O30 and Ba10Co0.25Ta7.9O30) and magnetic frustration (e.g. Sr2MIrO6; M = Ni and Zn, and Sr2MOsO6; M = Fe and Co). [4][5][6][7][8][9] Semiconducting or insulating magnetic perovskites (ferri-or ferromagnets) with high ordering temperatures are highly desirable for the use as spintronic devices. In this context, high ordering temperatures have been achieved recently in the double perovskites containing mixed 3d-4d or 5d metal ions at the B sites, e.g. Sr2FeMoO6 (TC =415 K), Sr2CrReO6 (TC =634 K), Sr2CrOsO6 (TC =725 K), and a quadruple perovskite CaCu3Fe2Re2O12 (TC = 560 K). [10][11][12][13] This is confirming that such materials offer a fertile ground to explore new perovskite-based materials with high transition temperatures. In our quest to explore materials with possible high magnetic transition temperatures, we have discovered an 3 interesting 8-layered twinned hexagonal perovskite that contains only Os at the B-site and is exhibiting above room temperature ferromagnetic response (TC ~ 430 K). So far, a twin option has not been observed in any 5d containing hexagonal perovskite.
SrO and SrO2 were mixed together and kept in one crucible whereas OsO2 was put into a separate crucible positioned underneath the SrOx crucible. Due to the decomposition of SrO2 above 650 K, OsO2 converts to gaseous OsO4 enabling reaction with the strontium oxides in the separate crucible. Prior to use, OsO2 was fully oxidized by heating it in presence of PbO2 (kept in separate crucibles) at 758 K for 48 h in an evacuated quartz tube. All the chemical manipulations were performed inside an Ar-filled glove box (O2 and H2O < 1 ppm), except for quartz tube sealing.
While heating, the tubes were always housed in a specially designed alumina casing which can be shut under static vacuum. This is to avoid any exposure of OsO4 to the environment in case of silica tube rupture. Thick hexagonal crystals with tapered edges of length up to ~100 μm were obtained as major product when reaction was carried at 1273 K. The crystals were sonicated in ethanol for separation from the rest of the mass and air dried. The product appears to be stable in air for weeks.
Caution: Tubes of sufficient thickness and length must be chosen in order to withstand the vapor pressure of OsO4 during high temperature reaction. Since OsO4, is highly toxic, silica tubes must be opened in a well-ventilated fume hood to avoid any potential contact with volatile OsO4. Proper personal protective equipment must be worn while working with osmium outside the glove box.
Single crystal structure determination. Single crystal X-ray diffraction (SCXRD) data sets were collected on an Apex-II CCD diffractometer (Bruker-AXS, Mo-Kα radiation, λ = 0.71073 Å, graphite monochromator) at ambient temperature using the APEX2 software suite. 14 A Lorentz and polarization factor and a numerical absorption corrections were applied. The structure was solved by charge flipping and the structure model was refined with SHELXL. 15,16 Crystallographic data, details of data collection and structure refinement, are given in Table 1 and atomic coordinates in Table 2. Selected bond lengths are given in Table 3 Table S1 in SI.
Magnetic property measurement. The magnetization of two independent samples of carefully selected loose crystals held in pre-calibrated quartz tube holders was measured on a SQUID magnetometer (MPMS-XL7, Quantum Design). The temperature dependence of magnetization, m(T) was recorded during warming cycle after zero-field cooling (zfc) and during field-cooling 5 (fc) under the applied magnetic fields 0H of 0.1, 3.5 and 7.0 T. The temperature range covered was 1.8-350 K and 300-750 K, without and with the instrument's oven insert, respectively. An isothermal magnetization curve was taken at T = 1.8 K. The susceptibility at infinite field was obtained using the simple Honda-Owen method. Before analyzing the data, the calculated sum of the diamagnetic increments for the compound (-512  10 -6 emu mol -1 , using the average of the values for Os 4+ and Os 6+ ) was subtracted. 18  were obtained making it difficult to separate manually. Use of as purchased OsO2 and SrO2 (in ratio 1:1 to 3:2) in separate crucibles lead to an almost equal mixture of crystals of cubic SrxOsO3 and the title phase, hexagonal Sr8Os6.3O24. At 1073 K only SrxOsO3 had formed as a secondary phase along with the target phase, however, the size of the crystals of the latter was quite small.

RESULTS AND DISCUSSION
At 1273 K the crystals of the desired phase were sufficiently large and clean to be manually picked, but a minor amount of impurity Sr2OsO5 and a new unknown phase always kept sticking. A phase pure sample was not obtained, even after several trials with ratios of starting materials and at different temperatures. Best sample containing the highest amount of single crystals of appropriate 6 size (~100  100 µm 2 ) and quantity sufficient for SCXRD, PXRD and magnetic measurements were obtained along the prescription as given in the experimental section.
The average composition of single crystals was determined using Scanning electron microscopyenergy dispersive X-ray analysis (SEM-EDX). An approximate Sr/Os ratio of ~ 1.27 was found for all the crystals ( Figure S1   R-values and residual electron density, but also atomic displacement parameters are concerned, a structure model with reduced symmetry might point to a somewhat more sophisticated occupation pattern. Several models, considering space groups P63, P3 ̅ c1 or P3c1, e.g., have been tested, but a stable model resolving these issues could not be obtained; all these refinements suffer from large correlations. Similar problems were previously encountered during the superstructure refinement 9 of a similar compound Sr4Ru3.05O12 which were then partially resolved by electron diffraction studies where spots corresponding to superstructure were clearly seen. 26 A cation disorder in the occupancy of FSO was also implied by electron microscopy, which prevented an absolutely accurate structure solution from X-ray diffraction, a situation also suspected to have occurred in our case. Thus, we consider our structure model presented here as an averaged one, but nevertheless quite approximate to the real structure, as few preceding reports of compounds with very similar structural features lend strong support to the validity of our structure model. [19][20][21]26 The resulting average composition Sr8Os6.3O24, is also compliant with the other reported B-site deficient hexagonal perovskites of similar structures (Ba8Ga2.4Ta4.96O24 and Sr4Ru3.05O12 = Sr8Ru6.1O24) and is in excellent agreement with the EDX results. 20,26 These vacancies at the FSO sites helps to reduce the electrostatic repulsion between the short Os−Os contacts thus stabilising     Magnetization. Temperature dependent magnetization data, m(T), for one of the two samples are shown in Figure 3a for two applied fields. Both the curves rise sharply below 430 K (the temperature of a magnetic phase transition) and tend towards saturation below  200 K. The temperature and field dependence below 430 K appears typical of a (weak) ferromagnetic material.
The fc and zfc curve diverge only slightly at low temperature, indicating soft ferromagnetism and reasonably low magnetic disorder. The isothermal magnetization, m(0H), at 1.8 K (Figure 3b) saturates at a low field of 0H  0.02 T and remains practically unchanged up to 7 T. The absence of a noticeable field hysteresis confirms the soft nature of the ferromagnetism. The saturated ferromagnetic moment per f. u. is small, only msat  0.8 B, similarly low ferromagnetic saturation moments in the magnetically ordered state have been observed for Ba2NaOsO6 and Ba11Os4O24. [32][33][34] The corrected inverse susceptibility 1/(T) is shown in Figure 3c. Due to the low sample masses  [10][11][12][13] However, all of them contain a combination of both 3d and 4d or 5d elements at the B sites, thus a high transition temperature is naturally expected.
Surprisingly, very recently in thin films of a cubic double perovskite Sr3OsO6 which contains only Os ion at the B-sites, extremely high TC of ~1064 K was reported which is the highest among any known semiconducting oxide. 35 However, the bulk sample lacks any magnetic order even down to 2 K. 36 Apparently, Sr8Os6.3O24 has the highest ordering temperature (430 K) among bulk perovskite compounds containing only 5d metal at B sites. Moreover it is the first example of an 8-layered hexagonal structure in 5d oxides exhibiting a twin option.