The YCa 4O(BO 3) 3 and YCa 4O(BO 3) 3: Yb; Yb and Er; Yb, Er and Ce crystals have been grown by the Czochralski technique. The YCa 4O(BO 3) 3 structure has been investigated by X-ray diffraction study. The refined composition of (Y,Ca)(Ca,Y) 2Ca 2O(BO 3) 3 is (Y 0.792Ca 0.208(6))(Ca 0.938Y 0.062(4)) 2Ca 2O 0.96(2)[ ] 0.04(BO 3) 3 ([ ]-oxygen vacancy). It was found that superstructure reflections with a doubled a-parameter are present for YCa 4O(BO 3) 3 and absent for doped crystals. The known and own structure data of LnCa 4O(BO 3) 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.