A NEW KOSTERS PRISM INTERFEROMETER FOR SIMULTANEOUS DETERMINATION OF REFRACTIVE INDEX AND THERMAL EXPANSION OF CRYSTALS AS A FUNCTION OF TEMPERATURE

摘要

An uniquely-modified Kosters prism interferometer for the simultaneous and independent measurement of refractive index, delta n(T), and thermal expansion, delta l(T), as a function of temperature in the range 308-1273 K, is described. The capabilities of the system are demonstrated via measurement of delta n(T) and delta l(T) along both the [100] and [001] directions up to and through the alpha-beta phase transition in quartz, with results in good agreement with previous measurements. Further to this, experiments to determine delta n(T) and delta l(T) along the [100], [010] and [001] directions in the orthorhombic nonlinear optical crystal indium-doped KTiOAsO4 are described. The results are used to show that the ferroelectric-paralectric phase transition at T-c = 1063 K is second-order and exhibits Landau-type behaviour over a range of 100 K below T-c. Finally, similar experiments on the orthorhombic crystal Ba2NaNb5O15 are reported. The behaviours of delta n(T) and delta l(T) at the ferroelastic phase transition at 563 K and the ferroelectric-paraelectric phase transition at T-c = 849 K are discussed. Clear evidence of the existence of polar fluctuations in the paraelectric phase is presented. [References: 34]