Hi Vineetha,
Refractive index of any semiconducting material can be approximated from its band gap using the following relationship:
n2 = 1 + [A/(Eg + B)]2
Where n is the refractive index, Eg band gap (eV) and A and B are constants amounting to 13.6 and 3.4 eV respectively.
The band gap itself can be determined either through diffuse reflectance spectroscopy using Kubelka-Munk theory (approximately) or Ultraviolet Photoelectron spectroscopy (accurate).
See the attached link for more details.
Article General relation between refractive index and energy gap in ...
https://wsu.edu/~pchemlab/documents/571-UPS-Lecture1.pdf
I hope this helps.
Presuming that you know the dielectric function of the ceramic and the size of the particles in the powder much smaller than the wavelength, you may try effective medium approach. Like Maxwell Garnett or Brugemann. If the size of the particles is comparable to the wavelength you'll get very complex multiple scattering and the refractive index won't be of much use.
I agree with the above directions, plus you can search for available articles in which relatively often you can find the appropriate values.
HI Vineetha,
I agree with aditya's answer. Also search old review articles or read "Elements of X-ray diffraction" text book by Cullity. Wish you good luck.
It's best to measure this property and there are a wide range of methods.
You do not tell us at what wavelength you require the refractive index (which is made up of 2 parts: the real part and the absorptive/imaginary component). You also do not tell us why - if it's for a laser diffraction experiment then you only need to real part to 2 decimal places and the imaginary to an order of magnitude (factor of 10). In the simplest analogy you could consider the KNN material to be made of sodium niobate and potassium niobate for which the optical properties are well known. At 632.8 nm (He-Ne laser) then for both the sodium and potassium niobates the RI is high (~ 2.10) and the imaginary low (say 0.001 or 0.01) - for laser diffraction you should conduct a robustness test on the angular intensity data. With dry laser diffraction there's less dependence on RI as the relative refractive index is much higher in air than in a fluid.
The data bellow was acquired using Ellipsometry (Sentech) by the Fraunhofer IKTS, Germany. I provided the disc-shaped solid samples which I fired in a Dekema 624 (oral design edition with heavy muffle) in accordance with the manufacturers recommendations. The data assumes the presence of porosities of around < 10 Vol.-% for an unknown solid using the Cauchy-Model. The Fraunhofer is Germanys premier scientific organisation for such matters. They worked out that ellipsometry was the method of choice.
Brechungsindex = Index of Refraction
Wellenlänge = Wavelength
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