Published: March 18, 2011 r2011 American Chemical Society 3559 dx.doi.org/10.1021/ic102477c | Inorg. Chem. 2011, 50, 3559–3566 ARTICLE pubs.acs.org/IC Structural Evolution and Properties of Solid Solutions of Hexagonal InMnO 3 and InGaO 3 Dmitriy A. Rusakov, † Alexei A. Belik,* ,† Stanislav Kamba, ‡ Maxim Savinov, ‡ Dmitry Nuzhnyy, ‡ Taras Kolodiazhnyi, § Kazunari Yamaura, § Eiji Takayama-Muromachi, †,§ Fedir Borodavka, ‡ and Jan Kroupa ‡ † International Center for Materials Nanoarchitectonics (MANA) and § Superconducting Materials Center (SMC), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Institute of Physics ASCR, Na Slovance 2, 18221 Prague 8, Czech Republic b S Supporting Information 1. INTRODUCTION Hexagonal manganites RMnO 3 (R = Sc, In, Y, and HoLu) crystallize in space group P6 3 cm and have a layered structure in which the layers of R 3þ ions are separated by layers of corner- shared MnO 5 trigonal bipyramids. 1,2 Mn 3þ ions form a triangular lattice producing strong frustration and resulting in interesting magnetic properties. 38 RMnO 3 shows antiferromagnetic long- range ordering with Neel temperatures, T N , of 130 K for ScMnO 3 and 7075 K for YMnO 3 . RMnO 3 also exhibits ferroelectricity with high ferroelectric Curie temperatures (T FE ) of 9001200 K. 1,913 Because of the high T FE , an investigation of the ferroelectricparaelectric phase transition by means of dielectric measurements is difficult because of the increased conductivity. The coexistence of ferroelectricity and magnetism makes RMnO 3 a multiferroic material. 1416 There are a lot of experimental and theoretical works on hexagonal RMnO 3 with R = Sc, Y, and HoLu. 122 Several scenarios have been suggested as the origin of the ferroelectric properties of YMnO 3 : (1) a displacement of the O1MnO2 axis of the MnO 5 polyhedron, 9 (2) an off-centering displacement of Y ions governed by electrostatic and size effects, 14 and (3) an off-centering displacement of Y ions governed by strong hybri- dization of Y 4dO 2p bonds. 1719 Detailed symmetry analyses for YMnO 3 show three possible paths from the high-temperature paraelectric P6 3 /mmc (Z = 2) structure to the low-temperature ferroelectic P6 3 cm structure (Z = 6), where Z is the number of formula units per unit cell. 9,20,21 Two intermediate phases have been suggested theoretically and experimentally: (1) an inter- mediate phase with P6 3 /mcm (Z = 6) symmetry 10,13 and (2) an intermediate nonferroelectric phase with P6 3 cm (Z = 6) symmetry. 9,14,21 The latest experimental works suggested the hybridization scenario 1719,21 and the nonferroelectric inter- mediate phase with P6 3 cm symmetry, P6 3 cm f P6 3 cm f P6 3 / mmc symmetry changes upon heating at T FE = 920 K and T npt = 1258 K, respectively, where T npt is a phase-transition tempera- ture corresponding to the cell tripling. 21 YMnO 3 is an improper ferroelectric with a polarization value of about 5 μC/cm 2 along the c axis. 14,20,22 It was suggested that the nonferroelectric P6 3 cm phase can be created after the K 3 Received: December 11, 2010 ABSTRACT: Solid solutions InMn 1x Ga x O 3 (0 e x e 1) have been investigated using magnetic, dielectric, specific heat, differ- ential scanning calorimetry (DSC), and high-temperature powder synchrotron X-ray diffraction (HT-SXRD) measurements. It was found that samples with 0.5 e x e 1 crystallize in space group P6 3 / mmc with a ∼ 3.32 Å and c ∼ 11.9 Å, and samples with 0.0 e x e 0.4 crystallize in space group P6 3 cm with a ∼ 5.8 Å and c ∼11.6 Å at room temperature. HT-SXRD data revealed the existence of a P6 3 cm-to-P6 3 /mmc phase transition at about 480 K in In- Mn 0.6 Ga 0.4 O 3 and at 950 K in InMn 0.7 Ga 0.3 O 3 . However, no dielectric, phonon, second-harmonic-generation, or DSC anoma- lies were found to be associated with these phase transitions. The phase transition should be improper ferroelectric from the sym- metry point of view, but the above-mentioned experimental facts, together with the absence of ferroelectric hysteresis loops, revealed no evidence for ferroelectricity in the low-temperature P6 3 cm structure. We suggest that InMn 1x Ga x O 3 corresponds to a nonferroelectric phase of hexagonal RMnO 3 with P6 3 cm symmetry. The antiferromagnetic phase-transition temperature decreases from 118 K for x = 0 to 105 K for x = 0.1 and 73 K for x = 0.2, and no long- range magnetic ordering could be found for x g 0.3. Specific heat anomalies associated with short-range magnetic ordering were observed for 0.0 e x e 0.5. InMn 1x Ga x O 3 with small Mn contents (0.8 e x e 0.98) has a bright-blue color.