dimer paramagnetic centers in lead germanate crystals doped with iron and halogen (Cl−, Br−, F−) ions

Abstract

Electron paramagnetic resonance (EPR) of trig onal Fe3+ centers (the ground state 6 S 5/2 ) in ferroelec tric lead germanate was studied in [1][2][3]. As a result, it was concluded that these centers are associated with individual trivalent iron ions localized in the position Pb7 (in the notation of [4]) of the ferroelectric phase Pb 5 Ge 3 O 11 . The local symmetry of the center increases in the paraelectric phase: C 3 C 3h . It was found in [5] that annealing of the above crystals in chlorine containing atmosphere leads to the appear ance of several Fe 3+ -Cl -dimer complexes with tri clinic symmetry, so that the chlorine ions are situated in the interstices of the channel (Fig. 1) formed by the Pb triangles and running along the C 3 axis [4,6]. This work is devoted to studying the mechanisms of the for mation of defects under doping of lead germanate by various halogen ions. 2.The measurements were performed on poly and single domain samples of Pb 5 Ge 3 O 11 single crystals grown with the use of the Czochralski method from the charge produced by solid phase synthesis and doped with 0.01-0.2 mol % Fe. The second order fer roelectric transition P3( ) ( ) in pure lead germanate occurs at a temperature of 450 K [7]; its crystalline structure at room temperature and at 473 K was found in [4,6] by neutron diffraction. To dope the crystals by halogen ions the samples were annealed at 800 K in the corresponding atmosphere for two hours. To prepare the chlorine , bromine , and fluorine con taining atmospheres in an open bulb we used ZnCl 2 , CsBr, and polytetrafluoroethylene (teflon), respec tively. The samples were made single domain by cool ing them down from 450 K in an electric field of 150 V/mm. The measurements were carried out on aBruker EMX Plus X band EPR spectrometer in the temperature range of 100-500 K.3. The EPR spectra in the region of the high field transition 5 6 (or 1/2 3/2, see Fig. 1 in [5]) of the Fe 3+ trigonal center in the initial and annealed Pb 5 Ge 3 O 11 samples at room temperature in the mag netic field B || C 3 are shown in Fig. 2. Annealing in the presence of zinc chloride (spectrum b in Fig. 2) leads to a considerable increase in the intensity of three sat ellites (marked by vertical arrows). In [5], these satel lites were attributed to the transitions of Fe 3+ -Cldimer complexes. It should be mentioned that the number of nonequivalent Fe 3+ -Cl -centers is actually more than 3, as indicated, e.g., by an increase in the line width of the trigonal complex, but we did not manage to resolve all of them. The presence of these centers in the initial crystal is caused by a small chlo ride contamination of germanium dioxide used in the charge synthesis.The temperature behavior of the positions of the signals in spectrum b (Fig. 2) is shown in Fig. 3. It proved impossible to measure the temperature depen dence for two satellites in the entire range owing to their overlap and strong (almost an order of magni tude) broadening at high temperatures. The bend in the temperature dependences near 435 ...

Raman scattering from the effective soft mode in lead germanate crystal

“…The crystal structure of lead germanate is a trigonal system with space group ( P 3) and three molecules in the elementary cell. The elementary cell has the following parameters: a = 1.0251 nm and c = 1.0685 nm . At T c = 451 K in this crystal, the second‐order ferroelectric phase transition of the type of displacement to the paraelectric phase occurs.…”

“…It consists of alternating layers of GeO 4 tetrahedra and double tetrahedra Ge 2 O 7 located along the ferroelectric axis. Moreover, its structure contains empty channels running along the edges of the unit cell, parallel to the optical axis . According to the results of the group‐theoretical classification of fundamental vibrations and the selection rules for the optical processes of the lead germanate lattice modes (point group‐ C 3 ), the optical representation for lead germanate is …”