Structural conditionality of proton conductivity inM_mH_n(XO₄)_(m+n)/2crystals

Abstract

Sb-Te binary compounds are well known as base materials for optical data storage media.They have long-period cubic close-packed ABC stacking structures [1].Two kinds of fundamental structural units form an intergroth along the stacking direction at the atomic level.Chemical formulae of Sb-Te binary compounds are expressed as (Sb 2 ) n (Sb 2 Te 3 ) m , where n and m are integers.The period of stacking structure is considered to be determined by the ratio of the two atomic species.However, our recent study revealed that stacking period of this system changes with temperature [2].Bi-Te binary compounds which have similar crystal structures also show this tendency.We then performed synchrotron powder diffratction experiments and crystal structure analyses in order to clarify the temperature dependence of these stacking period in Sb-Te and Bi-Te binary systems.Bi 14 Te 15 , Bi 10 Te 11 , Bi 8 Te 9 and Bi 3 Te 4 compounds were synthesized by melting method and Sb 67 Te 33 and Sb 60 Te 40 compounds were synthesized by sputtering method, as specimens.Powder X-ray diffraction experiments were carried out using a large diameter Debye-Scherrer camera with an imaging plate at the SPring-8 BL02B2 beamline.Temperature of powdered specimens were controlled by blowing hightemperature N 2 gas.Mesurements were carried out every 50 K from room temperature to 773 K.The specimens synthesized by sputtering method were amorphous and they crystallized with increasing temperature.It was found from the peak positions of diffraction pattern that the specimens have incommensurate structure at early stage of crystallization.Crystal structure analyses were performed under the superspace group approach, using JANA2006 [3].The basic unit cell consists of cubic close-packed three layers, and modulation of the atomic displacement is characterized by a single modulation vector q=γc*, where γ is a modulation period.In the case of Sb 67 Te 33 compound, as-deposited specimen was amorphous up to 388 K.It crystallized into an incommensurate structure with γ=1.633 at 423 K.The γ value then continuously changed with increasing temperature and drastically rised from 1.633 to 1.656 at around 700 K.However, it did not change in the following cooling down measurement.Final γ value 1.656 showed very good accordance with that expected from its composition.Similarly, as for all other specimens, changes of γ values and stacking structures were demonstrated.