396
T. Okura and K. Yamashita
composition 3] and substitution of rare earth ions [1 − 4(1 − x)/3=(4x − 1)/3] must
be accounted for in view of the N5-type crystal structure.
Banks et al. [8] have reported the values of σ 300 as 5 × 10 −3 to 1 × 10 −2 S/cm
for glass-ceramic Na 5 RSi 4 O 12 (R = Er, Y, Gd, Sm), which are as low as those of the
mixed-phase Narpsio specimens. The single-phase N5-type glass-ceramic was not
obtained in the present work. Based on the above crystallization analysis, their glassceramic specimens are reasonably considered to suffer from phase inhomogeneity
brought about by insufficient annealing. The formation of N5-type structure from
the precursor glasses is a matter of crystallization kinetics, because single-phase N5
has been synthesized in single crystal [2, 3, 33] or polycrystalline [6, 7, 11] form
based on the composition of N5. It is noted here that the precursor phases identified
were N3- or N9-type. Both N3 and N9 are considered to form isostructural [12, 21,
34] with Ca 3 Al 2 O 6 [35] to be comprised of the skeleton structure of six-membered
SiO 4 -tetrahedra rings [14]. It is generally known that phosphorus pentoxide acts
as a nucleating agent in the formation of glass-ceramics. It is therefore presumed
at present that the substitution of an asymmetric PO 4 -tetrahedron has a weakening
effect on the bonding of the skeleton structure of 6-membered SiO 4 -tetrahedra rings,
resulting in the tendency to form the stable 12-membered structure.
10.4.2 Conduction Properties of Crystalline Grains
The complex impedances and admittances of the measured Narpsio glass-ceramics
consisted of two semicircles below 300 ◦ C. The two intercepting points on the real
axis are interpreted as the resistance of the crystallized grains (R G ) and the total
resistance of grains and remaining glassy grain boundaries (R GB ). Figure 10.10
shows examples of the temperature dependence Arrhenius plots based on the calculated conductivity values of grains and grain boundaries of the glass-ceramics YNarpsio (Na 3.9 Y 0.6 P 0.3 Si 2.7 O 9 ) and Sm-Narpsio (Na 3.9 Sm 0.6 P 0.3 Si 2.7 O 9 ), in which
the geometrical ratios of thickness-to-surface area for grains were also used for
convenience for those of grain boundaries, because of their undefinable shapes.
Table 10.2 summarizes the measured conductivities (σ 300 ) and the calculated
activation energies (E a ) assigned for grains of the glass-ceramics with composition
7 of Sc to La, regardless of whether their crystalline phases are N5-type or not.
The conductivities, σ 300 , of single-phase Narpsio specimens of Er to Sc range
from 4 × 10 −2 to 1 × 10 −1 S/cm; in accordance the E a falls in the range of 23–
27 kJ/mol. In contrast, the mixed-phase Narpsio of Sc and In showed much smaller
σ 300 of 3 × 10 −3 with an E a of 35–40 kJ/mol, whereas non-Narpsio glass-ceramics
with unknown or mixed phases showed much lower conductivities of 1 × 10 −5 to
1 × 10 −4 S/cm with an E a of 55–58 kJ/mol.
The tendency of the conduction properties in single-phase Narpsio specimens is
consistent with the reported result measured on the corresponding polycrystalline
T. Okura and K. Yamashita
composition 3] and substitution of rare earth ions [1 − 4(1 − x)/3=(4x − 1)/3] must
be accounted for in view of the N5-type crystal structure.
Banks et al. [8] have reported the values of σ 300 as 5 × 10 −3 to 1 × 10 −2 S/cm
for glass-ceramic Na 5 RSi 4 O 12 (R = Er, Y, Gd, Sm), which are as low as those of the
mixed-phase Narpsio specimens. The single-phase N5-type glass-ceramic was not
obtained in the present work. Based on the above crystallization analysis, their glassceramic specimens are reasonably considered to suffer from phase inhomogeneity
brought about by insufficient annealing. The formation of N5-type structure from
the precursor glasses is a matter of crystallization kinetics, because single-phase N5
has been synthesized in single crystal [2, 3, 33] or polycrystalline [6, 7, 11] form
based on the composition of N5. It is noted here that the precursor phases identified
were N3- or N9-type. Both N3 and N9 are considered to form isostructural [12, 21,
34] with Ca 3 Al 2 O 6 [35] to be comprised of the skeleton structure of six-membered
SiO 4 -tetrahedra rings [14]. It is generally known that phosphorus pentoxide acts
as a nucleating agent in the formation of glass-ceramics. It is therefore presumed
at present that the substitution of an asymmetric PO 4 -tetrahedron has a weakening
effect on the bonding of the skeleton structure of 6-membered SiO 4 -tetrahedra rings,
resulting in the tendency to form the stable 12-membered structure.
10.4.2 Conduction Properties of Crystalline Grains
The complex impedances and admittances of the measured Narpsio glass-ceramics
consisted of two semicircles below 300 ◦ C. The two intercepting points on the real
axis are interpreted as the resistance of the crystallized grains (R G ) and the total
resistance of grains and remaining glassy grain boundaries (R GB ). Figure 10.10
shows examples of the temperature dependence Arrhenius plots based on the calculated conductivity values of grains and grain boundaries of the glass-ceramics YNarpsio (Na 3.9 Y 0.6 P 0.3 Si 2.7 O 9 ) and Sm-Narpsio (Na 3.9 Sm 0.6 P 0.3 Si 2.7 O 9 ), in which
the geometrical ratios of thickness-to-surface area for grains were also used for
convenience for those of grain boundaries, because of their undefinable shapes.
Table 10.2 summarizes the measured conductivities (σ 300 ) and the calculated
activation energies (E a ) assigned for grains of the glass-ceramics with composition
7 of Sc to La, regardless of whether their crystalline phases are N5-type or not.
The conductivities, σ 300 , of single-phase Narpsio specimens of Er to Sc range
from 4 × 10 −2 to 1 × 10 −1 S/cm; in accordance the E a falls in the range of 23–
27 kJ/mol. In contrast, the mixed-phase Narpsio of Sc and In showed much smaller
σ 300 of 3 × 10 −3 with an E a of 35–40 kJ/mol, whereas non-Narpsio glass-ceramics
with unknown or mixed phases showed much lower conductivities of 1 × 10 −5 to
1 × 10 −4 S/cm with an E a of 55–58 kJ/mol.
The tendency of the conduction properties in single-phase Narpsio specimens is
consistent with the reported result measured on the corresponding polycrystalline