The new cationic clathrates I Si30P16Te8-xSex and Si30+xP16-xTe8-xBrx were synthesized by the standard ampule technique. The Si30P16Te8-xSex (x = 0-2.3) clathrates crystallize in the cubic space group Pm (3) over barn with the unit cell parameter a ranging from 9.9382(2) to 9.9696(1) angstrom. In the case of the Si30+xP16-xTe8-xBrx (x = 1-6.4) clathrates, the lattice parameter varies from 9.9720(8) to 10.0405(1) angstrom; at lower Si/P ratios (x = 1-3) the ordering of bromine atoms induces the splitting of the guest positions and causes the transformation from the space group Pm (3) over barn to Pm (3) over bar. Irrespective of the structure peculiarities, the normal temperature motion of the guest atoms inside the oversized cages of the framework is observed. The title clathrates possess very low thermal expansion coefficients ranging from 6.6 x 10(-6) to 1.0 X 10(-6) K-1 in the temperature range of 298-1100 K. The characteristic Debye temperature is about 490 K. Measurements of the electrical resistivity and thermopower showed typical behavior of p-type thermally activated semiconductors, whereas the temperature behavior of the thermal conductivity is glasslike and in general consistent with the PGEC concept. The highest value of the thermoelectric figure of merit (ZT = 0.1) was achieved for the Br-bearing clathrate Si32.1(2)P13.9(2)Te6.6(2)Br1.0(1) at 750 K.