Microscopic structures of Laves phases and structurally related compounds: a transmission electron microscopy study

Abstract

We present microstructural investigations of Laves phases AB

Phases in the Al–Yb–Zn system between 25 and 50at% ytterbium

“…As already observed for other compounds crystallizing with the MgNi 2 -type in the systems CaMg 2Àx Al x [8] and CaZn 2Àx Al x [10][11][12][13], also for YbZn 0.996 Al 1.004 two characteristic aspects can be mentioned regarding the 6h site ðx; 2x; 1 4 Þ: a preferential occupation by the more electronegative element and a decreasing of the x parameter from the ideal value 0.1667, causing a distortion of the corresponding kagom e net. In this case the x parameter decreases to 0.1630 in the 6h site (Zn1/Al) mainly filled with zinc, allowing to realize a close Zn1/Al-2Yb2 contact of 3.236(1)Å, while maintaining the similar Zn1/Al-4Yb1 distance of 3.262(1)Å, which is practically insensible to the change of the x parameter.…”

“…increasing the aluminium content, the sequence MgZn 2 (C14)-MgNi 2 (C36)-MgCu 2 (C15) is always found in the CaLi 2Àx Al x [7], CaMg 2Àx Al x [8], CaAg 2Àx Al x [10] and YbZn 2Àx Al x systems. In other cases a Laves phase structure is lacking, but the sequence keeps its validity on increasing the valence electron concentration: MgZn 2 -MgNi 2 for SrMg 2Àx Al x [15], MgZn 2 -MgCu 2 for YbAg 2Àx Al x [9,14] and MgNi 2 -MgCu 2 for CaZn 2Àx Al x [10][11][12][13]. In five systems a binary term, namely SrAl 2 , CaAg 2 , CaZn 2 , YbAg 2 or YbZn 2 , belongs to the orthorhombic CeCu 2 -type.…”

“…therein) as end terms. Several series containing a dialuminide of Ca, Yb or Sr have been studied: CaM 2Àx Al x with M ¼ Li [7], Mg [8], Ag [9,10], Zn [10][11][12][13], YbAg 2Àx Al x [9,14] and SrMg 2Àx Al x [15]. In all systems a sequence of Laves phase structures is present with more or less large stability ranges.…”

Preparation, phase stability and structure of the C36 Laves phase Nb1–xCo2+x

“…No superstructure describing any kind of an ordered arrangement has been observed by X-ray diffraction and there is no evidence of an order-disorder transition to be seen in thermal analysis measurements. Moreover, TEM investigations including HREM, SAED, and CBED techniques on samples of the present study [37] exclude a superstructure. The substitution of the Nb atoms by the smaller Co atoms triggers a local displacement of the excess Co to the M1b and M2b positions (Fig.…”

“…Therefore, the configurational part of the entropy alone is not sufficient to stabilize C36 NbCo 3 (model IIIA) with respect to C36 NbCo 2 . Effects associated with the vibrational part of the entropy and stacking faults (which are readily seen in HREM images [37]) may be important in the stabilization of the title compound with respect to C36 NbCo 2 . However, the rough calculation above indicates that the C36 Nb 1Àx Co 2þx phase is stable only at high temperatures.…”