E.R. Andrievskaya/ Journal of the European Ceramic Sociery 28(2008)2363-2388 2369 2600 2600 2200 22 80Pr3 2600 2800 2400 2000 20001 mol mol T 2500 T,C 2300 L+H X C+H 2000 C+b 8 mol Fig 3. Phase diagrams of the binary systems with hafnia and lanthanide oxides from lanthana to gadolinia: (a)ZrO2-La203,63(b)ZrO2-Pr203, 63(c)ZrO2-Nd203, 63 (d)ZrO2-Sm203, 63(e)ZrO2-Eu2O3, (f) ZrO2-Gd2O3 63(O)experimental points: ()thermal analysis in air under solar furnace; (O)single-phase regions. (O gions by the data of XRd and annealing and quenching. cies in the anion sublattice of Ln2- Hf2+r07-(/2) decreases in solid solutions of fluorite type. 237 The thermodynamic stabil y/2.The pyrochlore-type phases demonstrate concentration ity of the pyrochlore type compounds can be determined using limits of stability, which are shown in Tables 4 and 5, Fig 8. empirical rule, which identifies the ratio between ionic radii The pyrochlore-type phases and their solid solutions are formed R=(r+Ln/+*HD), which must be higher than 1.2. In accordance y zirconia or hafnia with cerium subgroup of REO (from La with Colong, 235 the average ionic radi for the pyrochlore- to Gd), while the rEo of yttrium subgroup form disorder type phases can be approximately calculated by additive ruleE.R. Andrievskaya / Journal of the European Ceramic Society 28 (2008) 2363–2388 2369 Fig. 3. Phase diagrams of the binary systems with hafnia and lanthanide oxides from lanthana to gadolinia: (a) ZrO2–La2O3, 63 (b) ZrO2–Pr2O3, 63 (c) ZrO2–Nd2O3, 63 (d) ZrO2–Sm2O3, 63 (e) ZrO2–Eu2O3, (f) ZrO2–Gd2O3; 63 () experimental points; () thermal analysis in air under solar furnace; () single-phase regions, () two-phase regions by the data of XRD and annealing and quenching. cies in the anion sublattice of Ln2−xHf2+xO7−(x/2) decreases in ч /2.235 The pyrochlore-type phases demonstrate concentration limits of stability, which are shown in Tables 4 and 5, Fig. 8. The pyrochlore-type phases and their solid solutions are formed by zirconia or hafnia with cerium subgroup of REO (from La to Gd), while the REO of yttrium subgroup form disordered solid solutions of fluorite type.237 The thermodynamic stabil￾ity of the pyrochlore type compounds can be determined using empirical rule, which identifies the ratio between ionic radii R = (r3+ Ln/r4+Hf), which must be higher than 1.2. In accordance with Colong,235 the average ionic radii for the pyrochlore￾type phases can be approximately calculated by additive rule: