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Optical activity and critical exponent of the order parameter in lead germanate crystals. 
1. The case of diffused phase transition in Pb5Ge3O11 doped with Cu, Ba and Si ions
Adamenko D.I, Klymiv I.M, Vasylkiv Yu., Vlokh R.O.

Institute of Physical Optics, 23 Dragomanov St., 79005 Lviv, Ukraine

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In the present paper an approach for evaluation of critical exponent of the order parameter at the diffused phase transition is developed on the basis of optical activity data. The temperature dependence of the optical activity in Pb5Ge3O11:Cu2+ crystals is experimentally studied. The critical exponents obtained for Pb5Ge3O11:Cu2+ and Pb5(Ge0.6Si0.4)3O11 crystals are equal to 0.40 and 0.35, respectively. On the basis of the critical exponents evaluated it is shown that the solid solutions (Pb(1-x)Bax)5Ge3O11  with x = 0.05 stay quite close to tricritical point on the x,T-phase diagram (beta=0.28 ). The critical exponent for the pure lead germanate is also clarified.

Keywords: critical exponent, diffused phase transitions, optical activity, lead germinate crystals

PACS: 33.55.Ad, 77.80.Bh, 64.60.Fr, 77.84Bw, 78.20Wc, 61.72.Ww
UDC: 535.56
Ukr. J. Phys. Opt. 10 182-193 
doi: 10.3116/16091833/10/4/182/2009

Received: 22.10.2009

Анотація. В даній роботі розвинуто підхід для отримання значення критичного індексу параметру порядку при розмитих фазових переходах на основі даних про температурну поведінку оптичної активності. Експериментально досліджена температурна поведінка оптичної активності в кристалах Pb5Ge3O11:Cu2+. Отримані значення критичних індексів для кристалів Pb5Ge3O11:Cu2+  і   Pb5(Ge0,6Si0,4)3O11, які дорівнюють 0,40 та 0,35, відповідно. На основі отриманих значень критичних індексів показано, що тверді розчини (Pb(1-x)Bax)5Ge3O11 з x = 0,05 знаходяться близько до трикритичної точки на x,T- фазовій діаграмі (beta=0.28). Уточнене, також, значення критичного індексу для чистих кристалів германату свинцю.
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