Microwave dielectric constant of small samples: application to RNiO3 perovskites

We have developed a technique to measure the dielectric constant of small q uantities of compounds (a few milligrams) at microwave frequencies. When th e dielectric constant epsilon(r) is greater than 1, the electric field is g reatly modified inside the sample. The results have allowed us to overcome this problem when it is necessary to know the electric field in order to ex ploit further measurements such as conductivity or microwave Hall effect me asurements. As proof of the validity of our results on the value of the ele ctric microwave field, we have calculated the frequency shift for small sam ples in a microwave cavity. We have used classical results on the energy st ored in a microwave cavity, but with some corrections for which we have con sidered the corresponding elements between a microwave cavity and a low fre quency circuit. We have also made a proposal for a new method for measuring epsilon for very small samples (typically 1/100 mm(3)) using gallery mode cavities. We have given original results for epsilon in perovskites Sm1-xNd xNiO3 and some other components. (C) 1998 Elsevier Science Ltd. All rights reserved.