Role of non-stoichiometry in the cracking of oxide crystals

Tyagi

et al. 2009

Cryst. Res. Technol.

Single crystals of NaBi(WO 4 ) 2 (NBW) have been grown by Czochralski technique, employing differently synthesized starting charge and different axial temperature gradients. The causes behind inherent problems related to compositional changes and associated lowering in crystallization temperature have been probed by analyzing XRD and DTA patterns of post growth residual charge. Presence of low melting phases viz. Na 2 WO 4 and hitherto unreported compound Na 5 Bi(WO 4 ) 4 is thought to be responsible for the observed decrease in crystallization temperature. This problem was tackled by segregating pure phase material through re-crystallization, under high axial temperature gradient. The use of re-crystallized charge enabled transformation of almost the entire charge into a single crystal of high transparency. The effect of starting charge synthesis and temperature gradient on the optical transmission characteristics of NBW crystal has also been investigated.

Section: Discussioncontrasting

confidence: 88%

Effect of foreign phases on the growth of NaBi(WO₄)₂ crystals

Tyagi

et al. 2009

Cryst. Res. Technol.

“…An additional mechanism that may change the ε(T) function is the modification of the dipole moment of defect complexes in the process of thermal evolution [2]. Here the determining factor for changes in the sample is non-stoichiometry, whose role in cracking tungstate crystals was studied in [15].…”

Section: Resultsmentioning

confidence: 99%

Influence of thermal prehistory on the electrical properties of tungstate crystals

Shevchuk¹,

Kayun²

2011

Chem. Met. Alloys

Temperature-and time-induced changes of electric and dielectric parameters of AWO 4 (A = Cd, or Pb) single crystals were studied experimentally. The dielectric properties measured along the [001] direction in PbWO 4 single crystals (scheelite structure type) show a quasi-periodicity at about 20 h of free relaxation. A connection of this quasi-periodicity with structural and point defects dynamics, their interactions and the creation/rebuilding of complex defects (duration of the relaxation processes), is proposed. The main role in these phenomena is attributed to oxygen and lead vacancies. No thermodielectric effects were observed for CdWO 4 (wolframite structure type) perpendicular to the cleavage face (010) up to 500 K.

“…The non-stoichiometry manifests in different ways by influencing the optical and other physical properties of the crystal or it may promote the incorporation of impurities during crystal growth (Sabharwal et al 1996(Sabharwal et al , 2006Sabharwal andSangeeta 1999, 2008). The colouration of LGO-2 is related to Pb deficiency as borne out by the present measurements.…”

Section: Discussionmentioning

confidence: 99%

Optical and X-ray photoelectron spectroscopy of PbGeO3 and Pb5Ge3O11 single crystals

2010

Pb 5 Ge 3 O 11 crystals are found to exhibit pale yellow colouration while PbGeO 3 are colourless. X-ray photoelectron spectroscopy (XPS) measurements show lead deficiency in both the crystals. The results also reveal a stronger ionic character for PbGeO 3 as compared to Pb 5 Ge 3 O 11 crystal. The binding energy of Ge3d core level in the case of Pb 5 Ge 3 O 11 crystal is found to be smaller than the binding energy of germanium oxide, thereby indicating the incomplete oxidation of Ge ions in the crystal lattice. On gamma ray irradiation, the transmission of both the crystals is observed to deteriorate uniformly over the entire wavelength range, which has been attributed to the oxidation of some of the lattice Pb ions. On gamma irradiation the changes observed in O1s core level energies for both the crystals are seen to be consistent with the changes noted in the Pb4f 7/2 and Ge3d spectra. Interestingly, the results reveal oxidation of surface Ge atoms with atmospheric oxygen under gamma irradiation.