?Structural Transformations in Ceramics: Perovskite-like Oxides and Group III, IV, and V Nitrides?

Abstract

Ceramic perovskite-like oxides with the general formula (A A. . .)(B B. . .)O 3 and titanium-based oxides are of great technological interest because of their large piezoelectric and dielectric response characteristics.[1] In doped and nanoengineered forms, titantium dioxide finds increasing application as an organic and hydrolytic photocatalyst. The binary main-group-metal nitride compounds have undergone recent advancements of in-situ heating technology in diamond anvil cells leading to a burst of experimental and theoretical interest. In our DOE proposal, we discussed our unique theoretical approach which applies ab initio electronic calculations in conjunction with systematic group-theoretical analysis of lattice distortions to study two representative phase transitions in ceramic materials: (1) displacive phase transitions in primarily titanium-based perovskite-like oxide ceramics, and (2) reconstructive phase transitions in main-group nitride ceramics. A sub area which we have explored in depth is doped titanium dioxide electrical/optical properties. Work in displacive phase transitions in ceramic-like perovskite alloys has produced six papers in print (or press)[2, 3, 4, 5, 6, 7], one submitted paper[8], and two in preparation[9, 10], which can be grouped into seven main areas: (1) We have used domain average engineering symmetry methods to study heterogeneous domain structures in perovskite ferroelectrics. The domain average symmetry that we consider is applicable to crystals having a large number of domains. For such a case, the domains form a kind of nano-composite. The result[2] was a systematic derivation of all allowed "domain sets", their mesoscopic symmetry, information about the role of domain fractions in determining that symmetry, and the corresponding physical tensor properties for that domain set. The three cases we considered in depth corresponded to polarization (p x , p y , p z) oriented along [100], [110], and [111], resulting in the single domain state symmetries P 4mm, Amm2, and R3m, respectively. For the [111] polarization direction the PZM-PT and PMN-PT crystals are examples of interest. The well known BaTiO 3 is an example of a structural change with polarization along the [100] direction. KNbO 3 is an example of a material which undergoes a transition due to the spontaneous polarization toward the edge diagonals 110. As an example of the results we obtained, the dipole ordering along [100], corresponding to BaTiO 3 , is shown in Table 1. ISOTROPY contains the computer implementation of our algorithm for any space group to subgroup phase transition and is freely available[11]. (2) To validate these theoretical multidomain structures we have developed a fast thermodynamic approach to recreate mesoscopic multidomain ferroelectrics.[3] Using a phase field theory free energy functional we have developed a novel multi-order parameter evolution strategy for ferroelectrics is much faster than previous simulations employing instantaneous mechanical equilibrium[12, 13, 14, 15, 16, 17, 18, 19]....