Synthesis,
Crystal Structure, and High-Temperature
Phase Transition of the Novel Plumbide Na<sub>2</sub>MgPb
Takahiro Yamada
Takuji Ikeda
Ralf P. Stoffel
Volker
L. Deringer
Richard Dronskowski
Hisanori Yamane
10.1021/ic500466w.s002
https://acs.figshare.com/articles/dataset/Synthesis_Crystal_Structure_and_High_Temperature_Phase_Transition_of_the_Novel_Plumbide_Na_sub_2_sub_MgPb/2290813
A hitherto
unknown sodium magnesium plumbide, Na<sub>2</sub>MgPb, was synthesized
by heating the constituent elements. Na<sub>2</sub>MgPb crystallizes
in a hexagonal unit cell with the Li<sub>2</sub>CuAs-type structure
(<i>P</i>6<sub>3</sub>/<i>mmc</i>, <i>Z</i> = 2, <i>a</i> = 5.110(2) Å, <i>c</i> =
10.171(4) Å at 293 K). The compound furthermore displays polymorphism:
high-temperature powder XRD measurements revealed that hexagonal Na<sub>2</sub>MgPb (dubbed the “α” phase) transforms
to another hexagonal phase (β) which is existent at 493–553
K, and the β phase changes to a cubic structure (γ) at
533–633 K further. The molar volume of γ-Na<sub>2</sub>MgPb is approximately 9% and 13% smaller than the molar volumes of
the α phase and the β phase, respectively (at 543 K).
The electrical resistivity of Na<sub>2</sub>MgPb is 0.39 mΩ
at 300 K; it rises with increasing temperature from 300 to 491 K,
and then drops at 491 and 523 K. These abrupt changes in resistivity
may be attributed to the α → β and β →
γ phase transitions, respectively. To gain further insight into
the structure of cubic γ-Na<sub>2</sub>MgPb, putative models
with regular Heusler-type (Cu<sub>2</sub>MnAl-type) and inverse Heusler-type
(Li<sub>2</sub>AgSb-type) arrangements were probed using first-principles
computations based on density functional theory (DFT). These computations
indicate that, for the cubic γ phase, an inverse Heusler-type
structure is distinctly more stable than the alternative regular Heusler
type (at 0 K); beyond that, <i>ab initio</i> thermochemistry
was successfully used to verify the stability ordering (α-Na<sub>2</sub>MgPb being favorable at low temperature, γ-Na<sub>2</sub>MgPb at high temperature), albeit the theoretically predicted transition
temperature of 900 K which is higher than observed in experiment.
2014-05-19 00:00:00
β phase changes
Li 2AgSb arrangements
ab initio thermochemistry
0.39 m Ω
Na 2MgPb crystallizes
DFT
Li 2CuAs structure
Na 2MgPb
Novel Plumbide Na 2MgPbA
powder XRD measurements
sodium magnesium plumbide