Gadolinium calcium oxide borate GdCa₄O(BO₃)₃ (GdCOB) crystals have monoclinic, noncentrosymmetric symmetry, and, therefore, exhibit nonlinear effects. Especially they are potential effective generator of the 2nd and higher harmonics of electromagnetic radiation. However, we have discovered that many crystals owe a core region situated in their central parts. In contrary to e.g. YAG crystals, when the core can be seen with a naked eye, the core in GdCOB is visible only in polarized radiation ( this can be also evidenced by the X-ray techniques).

A question arises on the nature of the core and its potential use (or disuse) in optics and optoelectronics. In this work we report our primary results concerning the optical homogeneity around the core and in the core region itself. The core in many cases can consume up to 20 – 25% of the crystal diameter.

The GdCOB melts congruently and can be grown by the Czochralski method. Single crystals with diameters up to 35 mm and lengths up to 50 mm were grown from the stoichiometric ratio of Gd₂O₃, CaCO₃ and B₂O₃. The powders were prepared by solid - state reaction. The crystals were grown in nitrogen atmosphere from a 100 mm dia. iridium crucible using the [010]–oriented seed. The pulling rate was in the range of 0,8 – 1 mm/h, and the rotation speed was adjusted to 20 rpm.

The as-grown crystals were colourless and their good optical homogeneity was evidenced by optical ( spectroscopic, polariscopic ) and X-ray techniques (X-ray Lang projection topography in transmission and back reflection geometry).

We have observed two types of the core, namely the one with large gradients of birefringence, and the other one when the birefringence was fairly constant, compared with the neighbouring regions in the crystals. However, the latter case was found to be predominant. It is worth noting that this phenomenon, i.e. that the core was found to be the most homogeneous part of the crystals, was evidenced accordingly by the optical and X-ray techniques. Therefore, we can conclude that the core in GdCOB might be also of a potential use in optical applications.

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