DOI: 10.1002/zaac.200700588 Synthetic, Structural and Magnetic Studies on Chromium Orthoborate Ömer F. Öztürk a , Birgül Zümreoglu-Karan a, *, Musa M. Can b , and Sadan Ozcan b Ankara / Turkey, Hacettepe University, a Department of Chemistry and b Department of Physics Engineering Received October 23 rd , 2007; accepted January 3 rd , 2008. Abstract. Effects of various Cr and B sources and preheating con- ditions on the high temperature synthesis of chromium orthoborate were studied. Precursors were mixed, ground, heated at 1000 °C and reground. The obtained products were checked by XRD, vi- brational and electronic spectroscopy, SEM and density measure- ments. Some characteristic properties were redetermined and com- pared with the earlier data. The reaction between hydrated Cr(III) sulfate and boric acid (or hydrated ammonium borate salts) was 1 Introduction Advanced borate materials have found a wide utilization due to their diverse electronic, magnetic and optical proper- ties [1]. Many ternary and quaternary alkali and earth alka- line metal borates execute good quality performances as non-linear optics and laser sources [2]. Transition metal borates, in spite of the interest in their electrochemical and magnetic properties, have been less comprehensively studied when compared to alkali metal borates [3]. Among the calcite-type transition metal orthoborates, MBO 3 (M = Fe, V, Cr, Ti) [4], where discrete trigonal borate units are em- bedded in the metal oxide framework, CrBO 3 still remains poorly explored. Only four reports have been published so far for the synthesis of chromium orthoborate. Tombs et al. first reported the preparation of CrBO 3 in 1963 [5], by the reaction between Cr 2 O 3 and B 2 O 3 in molten B 2 O 3 at 1100 °C for 12h. Seven years later, Bither et al. [6] reported that the mixture 4CrCl 3 + 6Na 2 B 4 O 7 + 10B 2 O 3 yields CrBO 3 by a flux reaction at 700 °C for 12h. After a gap of nearly three decades in the literature, Kuznetsov reported in 2001 [7], a one-step combustion synthesis of CrBO 3 from Cr + 0.5Cr 2 O 3 + B 2 O 3 by employing NaClO 4 as a solid oxidizer. Balaev et al. [8] prepared a single crystal CrBO 3 from melt solutions of Cr 2 O 3 -B 2 O 3 -(70 wt % PbO + 30 wt % PbF 2 ). A different type of chromium(III) borate with the norbergite structure, Cr 3 BO 6 , was synthesized in high purity by using a hydrated Cr III sulfate precursor in 2001 [9]. * Birgül Zümreoglu-Karan Hacettepe University Faculty of Science, 06800 Ankara / Turkey Tel:+90 312 297 79 51; Fax:+90 312 299 21 63 E-mail:bkaran@hacettepe.edu.tr Z. Anorg. Allg. Chem. 2008, 634, 1127-1132  2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1127 found to be the most effective in providing nanosize crystallites of calcite-type CrBO 3 with 99% purity. Finally, temperature depen- dent magnetic properties were investigated and paramagnetic to antiferromagnetic transition was observed at 12 K. Keywords: Chromium; Chromium borate; High temperature synthesis; Crystal structures; Magnetic properties Magnetic behavior of calcite-type transition metal ortho- borates has not been fully understood due to the limited research on these materials and also to the complexity of their magnetic structure. Temperature dependent magnetiz- ation of CrBO 3 was first reported by Bither et al. in 1970 [6]. No field dependence was observed when the magnetiz- ation was measured using a field gradient of 10 7 Oe 2 cm -1 at 8 kOe and the sample was found antiferromagnetic with a Neel temperature, T N , of 15 K. The magnetic exchange mechanism was claimed to be mediated via the intervening oxide anions which is known as the “superexchange mecha- nism” [10]. After 33 years, the measurement was repeated by Balaev et al. in 2003 [8]. The single crystal CrBO 3 sample was shown to exhibit not only axial but also hexagonal an- isotropy. The authors reported that they could not fit the data to a simple model of collinear two-sublattice antiferro- magnet with magnetic moments aligned with the [111] axis, as Bither et al. did [6]. The scarcity in the published data on chromium borates arises from the complications in the synthetic procedure. High temperature synthesis is required to achieve a stable CrBO 3 phase, however, B 2 O 3 gradually evaporates around 1000 °C which necessitates the use of boron oxide in excess. On the other hand, CrBO 3 decomposes to Cr 2 O 3 and B 2 O 3 at 1220 °C which limits the temperature range of the solid state reaction. It is well known that the nature of the pre- cursors and processing conditions are crucial to optimize the properties of the final material in the conventional solid state technique. Materials with better performance charac- teristics can be developed by refining/controlling the pro- cess conditions. In the present work, we report the influence of various chromium precursors (metallic Cr, Cr 2 O 3 , Cr 2 (SO 4 ) 3 · 15H 2 O) and boron sources (crystalline B, B 2 O 3 , (NH 4 ) 2 B 4 O 7 ·4H 2 O, (NH 4 ) 2 B 10 O 16 ·8H 2 O, H 3 BO 3 ) on chro- mium orthoborate synthesis. The correlations between the structural, physical and magnetic properties and the prep-