Na₂Ge₃P₃ and Na₅Ge₇P₅ Comprising Heteroatomic Polyanions Mimicking the Structure of Fibrous Red Phosphorus

Abstract

Recently lithium phosphidogermanates were discovered as fast lithium ion conductors for potential usage as solid electrolytes in all solid‐state batteries. In this context we also studied sodium phosphidogermanates since sodium ion conductors are of equal interest. Na2Ge3P3 and Na5Ge7P5 both crystallize in the monoclinic space group C2/m with unit cell parameters of a = 17.639(4) Å, b = 3.6176(7) Å, c = 11.354(2) Å, β = 92.74(3)° and a = 16.168(5) Å, b = 3.6776(7) Å, c = 12.924(4) Å, β = 91.30(3)°, respectively. Both show linearly condensed 9‐atom cages of four Ge / five P and five Ge / four P atoms, respectively. These cages contain Ge–Ge bonds and form one‐dimensional tubes by sharing three atoms. The parallel tubes are paired through further Ge–Ge bonds. Both structures are closely related to the one of the fibrous type of crystalline red phosphorus. A comparison with other compounds such as NaGe3P3 and GeP reveals recurring structural motifs with a broad variety of connection patterns. According to the general formula Na4+xGe6+xP6–x with x = 0 and 1, the two novel structures hint for the possibility of a variable Na content which might allow Na ion mobility.

Na3Ge2P3: A Zintl Phase Featuring [P3Ge–GeP3] Dimers as Building Blocks

“…The Ge−Ge bond length of 2.4885(5) Å is slightly elongated compared to those found in elemental Ge (2.450 Å) 54 and is in the same range as in structurally related ternary compounds such as NaGe 3 P 3 (Ge−Ge, 2.626 Å) or Na 2 Ge 3 P 3 (Ge−Ge, 2.463 Å). 36,37 Further on, Ge−P bond lengths of 2.3191(9)− 2.3607(10) Å in the Ge(GeP 3 ) tetrahedra are in good agreement with Ge−P distances found in other compounds of the same ternary system. 30,36,37 Interestingly, bond angles of 96.04(3)−122.14(3)°show relatively high deviation from the ideal tetrahedron angle of 109.47°.…”

“…36,37 Further on, Ge−P bond lengths of 2.3191(9)− 2.3607(10) Å in the Ge(GeP 3 ) tetrahedra are in good agreement with Ge−P distances found in other compounds of the same ternary system. 30,36,37 Interestingly, bond angles of 96.04(3)−122.14(3)°show relatively high deviation from the ideal tetrahedron angle of 109.47°. The Na atoms are located in cavities of the two-dimensional polyanions as well as in between the layers.…”

“…61 Formal addition of two Na results in Na 2 Ge 3 P 3 where the two-dimensional anionic partial structure is cleaved into one-dimensional 1 ∞ [Ge 3 P 3 ] 2− strands. 36 In detail, the [Ge 3 P 3 ] building blocks from NaGe 3 P 3 are maintained but show different connection, as Ge−Ge dimers are now connected forming Ge 4 units and a (2b-P) 1− bridge adds an additional negative charge to the polyanion (Figure 3f). Here, the anionic partial structure can be described as strands of corner-sharing GeP 4 tetrahedra which are connected by rows of parallel Ge 4 chains.…”

“…Here, the anionic partial structure can be described as strands of corner-sharing GeP 4 tetrahedra which are connected by rows of parallel Ge 4 chains. Another compound with the same connection pattern but different composition, namely Na 5 Ge 7 P 5 , 36 results from a partial substitution of a P atom by Ge which extends the Ge 4 chain and adds another Na atom position. Thus, the compound does not appear on one of the lines of the composition diagram described above.…”

“…35 Just recently, two new compounds, Na 2 Ge 3 P 3 and Na 5 Ge 7 P 5 , in the system of sodium phosphidogermanates were reported. 36 Both compounds feature a complex polyanionic framework including homoatomic Ge−Ge bonds. A structural comparison of these compounds with previously reported NaGe 3 P 3 , 37 that shows a similar 2D polyanionic structure, or Na 10 Ge 2 P 6 , 30 which features isolated dimeric building units, reveals that the Na−Ge−P system is very promising to 38 or clathrate compounds 39 hints for a plethora of possible structure motifs.…”