Gd2Sn2O7 and Gd2Ti2O7 have been regarded as good experimental realizations of the classical Heisenberg pyrochlore antiferromagnet with dipolar interaction. The former was found to adopt the Palmer-Chalker state via a single, first-order transition at TN≈1K, while the latter enters a distinct, partially ordered state through two successive transitions at TN1≈1K and TN2=0.75K. To shed more light on their distinct magnetic ground states, we have synthesized two more gadolinium-based pyrochlore oxides, Gd2Ge2O7 and Gd2Pt2O7, under high-pressure conditions and performed detailed characterizations via x-ray powder diffraction, dc and ac magnetic susceptibility, and specific heat measurements down to 100 mK. We found that both compounds enter a long-range antiferromagnetically ordered state through a single, first-order transition at TN=1.4K for Gd2Ge2O7 and TN=1.56K for Gd2Pt2O7, with the specific heat anomaly similar to that of Gd2Sn2O7 rather than Gd2Ti2O7. Interestingly, the low-temperature magnetic specific heat values of both Gd2Ge2O7 and Gd2Pt2O7 were found to follow nicely the T3 dependence as expected for a three-dimensional antiferromagnet with gapless spin-wave excitations. We have rationalized the enhancement of TN in terms of the reduced Gd-Gd distances for the chemically pressurized Gd2Ge2O7 and the addition of extra superexchange pathways through the empty Pt-eg orbitals for Gd2Pt2O7. Our current study has expanded the family of gadolinium-based pyrochlores and permits us to achieve a better understanding of their distinct magnetic properties in a more comprehensive perspective.
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