The crystal structure and dielectric properties of LaYbO3 ceramics prepared by the mixed-oxide route have been investigated. Rietveld refinements performed on X-ray and neutron diffraction data show the room-temperature structure to be best described by the orthorhombic Pnma space group [a- 6.02628(9) angstrom, b = 8.39857(11) angstrom, and c = 5.82717(7) angstrom; Z = 4, and theoretical density, D-x = 8.1 g/cm(3)] in agreement with electron diffraction experiments. LaYbO3 ceramics. red at 1600 degrees C for 4 h attain similar to 97% of D-x and their microstructures consist of randomly distributed equiaxed grains with an average size of similar to 8 mu m. Conventional transmission electron microscopy shows densification to occur in the absence of a liquid phase and reveals domain-free grains. The relative permittivity, epsilon(r), of LaYbO3 ceramics at radio frequencies is similar to 26 in the range similar to 10-300 K; however, a small dielectric anomaly is detected at similar to 15 K. At room temperature and microwave frequencies, LaYbO3 ceramics exhibit epsilon(r) similar to 26, Q x f(r)similar to 20 613 GHz (at 7 GHz), and tau(f) similar to -22 ppm/ K. Q x f(r) show complex subambient behavior, decreasing from a plateau value of similar to 20 000 GHz between similar to 300 and 200 K to a second plateau value of similar to 6000 GHz at similar to 90 K before decreasing to < 1000 GHz at similar to 10 K. The large decrease in Q x f(r) at low temperature may be related to the onset of antiferromagnetism at similar to 2.7 K.