FeF, and Mn2Sb207 elhi it 7 ex ibit long-range magnetic order at 1 5.5 netic susceptibility data fo th or ese compounds show a stron de r er at .5 and 13 K, respectively. B lk u magations from a Curie-Wei -1 b h ssaw e avior far above the orderin tern a s rong ependence on sample histor d d y an evlbuted to short-range order e or ering temperatures. These effects a tt ' d b h }1' hl fr tdl tti g a e attlce of corner-sharin tetr above T eu ronl raction data given clear evidence T, . Fo ' 1 i ofth d t ' ld F in erms o spin-spin corr 1 t' e a ions out to the f u ion unction g o oo to s e serve strong antiferromagnetic first-nei hbor n ir -neighbor, spin-spin correlations.ig or, and ferromagnetic second-

The room-temperature reflectance of a well-characterized series of samples (x-ray, neutron activation and thermigravimetric analyses, resistivity, magnetization) in the Lal "Ti03 system has been measured between 50 and 40000 cm on samples ranging from the antiferromagnetic insulating (LaTi03) to the metallic (Lao 88Ti03}part of the phase diagram. The electronic portion of the low-frequency optical conductivity increases with frequency at the lowest frequencies, similar to several barely metallic systems.This non-Drude behavior can be modeled as the sum of the two low-frequency oscillators, a Drude contribution that increases systematically with doping and a broad midinfrared continuum. The midinfrared band, which Inay be associated with transitions across the Hubbard gap, persists in highly doped samples in agreement with theoretical predictions. If one assumes a single low-frequency component with frequency-dependent scattering rate, one finds a negative mass enhancement below 150 cm ' in metallic samples close to the metal-insulator phase boundary.