Structure transformations and dielectric properties of PbY_1/2Nb_1/2O₃ and PbHo_1/2Nb_1/2O₃ compounds

Abstract

The metastable pseudomonoclinic perovskite phases, PbY_1/2Nb_1/2O₃ and PbHo_1/2Nb_1/2O₃ were prepared under conditions of high pressures and temperatures. Investigations of structural parameters, thermal stability and some dielectric properties were carried out. The data were analyzed by comparison with corresponding data of other representatives of the PbB_1/2³⁺Nb_1/2O₃ series. The dependence of unit cell parameters on the B³⁺ rare earth cation radius for the PbB_1/2³⁺Nb_1/2O₃ perovskites was correlated with a change of electronic structure of the rare earth elements, in particular, with the 4f-shell filling.

Introduction

Compounds of the PbB_1/2³⁺Nb_1/2O₃ series have a special place among complex lead-containing perovskites. Depending on the B³⁺ cation nature, the present series is characterized by a variety of peculiarities in crystal structure and physical behavior [1].

Phase transitions between polymorphs of different structure and dipole character were found for representatives of the series. Compounds with small B³⁺ cations have a statistical cation distribution over the octahedral sublattice and are, as a rule, ferroelectrics with a diffuse phase transition [1]. As the size of B³⁺ cation increases, a tendency to increasing degree of crystal lattice distortion, ordering of octahedral cations and increasing the temperature of the phase transition to the dipole ordered state (T_C) is observed. For complex lead niobates with rare earth cations, a pseudomonoclinic lattice distortion with B-cation ordering is characteristic. As a rule, they all rank among the group of antiferroelectrics with high T_C. It has been recently revealed [2] that these compounds may serve as components of solid solutions with a morphotropic phase boundary.

Some compounds of the PbB_1/2³⁺Nb_1/2O₃ series are known to crystallize in a pyrochlore-type structure on synthesis by a conventional ceramic technique. Nevertheless, under high pressure and temperature, a polymorphic transformation into a perovskite structure occurs, which is retained as a metastable phase under normal pressure. Obtaining such metastable phases extends the family of lead-containing perovskite series of this type [3], [4], [5], [6], [7] and allows to observe the dependence on B³⁺ cation radius (R_B) of the average unit cell parameter (a_av), the temperature of dielectric permittivity maximum (T_m) and the change of unit cell volume at the perovskite–pyrochlore transition (ΔV/V).

In particular, these dependences have enabled the prediction of properties of a group of compounds with the perovskite structure that have not been investigated so far. Analysis of the a_av(R_B) and T_m(R_B) plots has initiated a definition of structural and dielectric parameters of these perovskite series which deviate from the common pattern. On the basis of investigation of the ΔV/V(R_B) dependence, it was determined that the pyrochlore–perovskite phase transition of compounds with small size B³⁺ cations is accompanied by a much larger decrease of the specific unit cell volume than that of compounds with large rare earth cations. The need for high pressure during synthesis of the latter compounds can probably be associated with their instability at high temperatures, due to different temperature dependence of the thermal expansion coefficients of the perovskite and pyrochlore phases [7].

Information about a possible perovskite phase of composition PbY_1/2Nb_1/2O₃ (PYN) [8] is absent. For this composition, a predicted decrease of the specific unit cell volume, when changing from pyrochlore to perovskite structure, is close to a “critical” value (∼3%) for the PbB_1/2³⁺Nb_1/2O₃ series [7]. So, we may suppose that lead yttrium niobate may be obtained in a perovskite structure by polymorphic transformation from a pyrochlore phase under high pressure.

Reports about the possibility of PbHo_1/2Nb_1/2O₃ (PHN) perovskite synthesis by the conventional ceramic route are contradictory. Some authors [9] reported preparation of a perovskite phase at atmospheric pressure and its dielectric properties. However, it was later noted that lead holmium niobate could not be synthesized with a perovskite structure, either by the conventional technique [10] or by hot pressing [2]. The lead niobates with nearest neighbours to Ho in the periodic table—Er and Dy in the B-sublattice—were obtained with the metastable perovskite structure only under high pressure [5], [6]. The possibility of obtaining PbEr_1/2Nb_1/2O₃ perovskite by addition of excess PbO and by hot pressing was also reported [2]. The practice of adding excess lead oxide is wide spread, but recently, in connection with a thorough investigation of the so-called relaxor state, attention was attracted to a considerable influence of deviation from stoichiometric PbO content in the compounds on the dielectric response of the associated perovskite phases [11].

In the present work, the metastable perovskite phases, PYN and PHN are obtained using high-pressure synthesis, temperature limits of their stability are determined, and structural and dielectric characteristics investigated in comparison with other compounds of the PbB_1/2³⁺Nb_1/2O₃ series.