  We present here magnetization, specific heat and Raman studies on
single-crystalline specimens of the first pyrochlore member $Sm_2Ti_2O_7$ of
the rare-earth titanate series. Its analogous compound $Sm_2Zr_2O_7$ in the
rare-earth zirconate series is also investigated in the polycrystalline form.
The Sm spins in $Sm_2Ti_2O_7$ remain unordered down to at least T = 0.5 K. The
absence of magnetic ordering is attributed to very small values of exchange
($\theta_{cw} ~ -0.26 K$) and dipolar interaction ($\mu_{eff} ~ 0.15 \mu_B$)
between the $Sm^{3+}$ spins in this pyrochlore. In contrast, the pyrochlore
$Sm_2Zr_2O_7$ is characterized by a relatively large value of Sm-Sm spin
exchange ($\theta_{cw} ~ - 10 K$); however, long-range ordering of the
$Sm^{3+}$ spins is not established at least down to T = 0.67 K, due to
frustration of the $Sm^{3+}$ spins on the pyrochlore lattice. The ground state
of $Sm^{3+}$ ions in both pyrochlores is a well-isolated Kramer's doublet. The
higher-lying crystal field excitations are observed in the low-frequency region
of the Raman spectra of the two compounds recorded at T = 10 K. At higher
temperatures, the magnetic susceptibility of $Sm_2Ti_2O_7$ shows a broad
maximum at T = 140 K while that of $Sm_2Zr_2O_7$ changes monotonically. Whereas
$Sm_2Ti_2O_7$ is a promising candidate for investigating spin-fluctuations on a
frustrated lattice as indicated by our data, the properties of $Sm_2Zr_2O_7$
seem to conform to a conventional scenario where geometrical frustration of the
spin exclude their long-range ordering.
