The YCa4O(BO3)(3) and YCa4O(BO3)(3): Yb; Yb and Er; Yb, Er and Ce crystals have been grown by the Czochralski technique. The YCa4O(BO3)(3) structure has been investigated by X-ray diffraction study. The refined composition of (Y,Ca)(Ca,Y)(2)Ca2O(BO3)(3) is (Y0.792Ca0.208(6))(Ca0.939Y0.062(4))(2)Ca2O0.96(2)[](0.04)(BO3)(3) ([]-oxygen vacancy). It was found that superstructure reflections with a doubled a-parameter are present for YCa4O(BO3)3 and absent for doped crystals. The known and own structure data of LnCa(4)O(BO3)(3) crystals can be used to obtain a relation between mean interatomic cation-anion distances of (Ln,Ca)(1) and (Ca,Ln)(2) sites and rare-earth (Ln) elements distribution in cation positions (x(1) and x(2)) upon the ionic radii of Ln. Similar equations were derived for dependence of unit cell a, b, c parameters upon the "radius" of the (Ln,Ca)(1) and (Ca,Ln)(2) cation positions The equations enable the distribution of Y, Ca and dopants on cation sites to be proposed. (C) 2002 Elsevier Science B.V. All rights reserved.