Developments in Ceramic Materials Research

134
José M. Rojo, José L. Mesa and Teófilo Rojo
pathways involve one phosphate group with the following O-P-O angles: i) Cr(1)-O-P-O-
Cr(2) with values of 113 and 120°, respectively; ii) Cr(1)-O-P-O-Cr(3) with 111 and 118° and
Cr(2)-O-P-O-Cr(3) with 113 and 114°; iii) finally, the Cr(1)-O-P-O-Cr(1); Cr(2)-O-P-O-Cr(2)
and Cr(3)-O-P-O-Cr(3) pathways exhibit angles of 108, 122 and 101°, respectively.
Table 4. Selected geometrical parameters, bond lengths (Å) and angles (°) related to the
possible magnetic superexchange pathways for M(PO3)3 (MIII= Mo and Fe) and
Cr2(P6O18)
Angles (°)
M(PO3)3 Direct distance Length of exchange M-O-P O-P-O P-O-M
M-M (Å) pathway (Å)
M(1)-O-P -O-M(3) Mo (AF) 5.58 7.08 149.2(6) 116.0(6) 139.3(6)
(Solid lin e in Fig. 31) Fe (AF) 5.28 6.94 147.4(4) 116.9(3) 136.7(3)
M(1)-O-P -O-M(3) Mo (AF) 5.94 7.22 138.4(6) 121.1(6) 158.0(7)
(Solid Lin e) Fe (AF) 5.36 6.95 141.4(3) 117.8(4) 155.7(4)
M(2)-O-P -O-M’(2) Mo (AF) 5.40 7.14 144.4(6) 119.7(6) 154.4(6)
(Solid lin e) Fe (AF) 5.32 6.89 147.2(4) 120.7(4) 149.5(4)
M(1)-O-P -O-M’(1) Mo (F) 5.75 7.15 139.5(7) 118.1(6) 133.3(5)
(Point line) Fe(AF) 5.57 6.87 147.4(4) 111.4(3) 135.7(3)
M(1)-O-P -O-M(2) Mo (AF) 5.37 7.10 135.4(5) 111.1(5) 140.5(7)
(Point line) Fe (AF) 5.33 6.98 131.6(5) 115.2(2) 140.2(3)
M(1)-O-P -O-M(2) Mo (F) 5.44 7.15 140.5(7) 116.1(5) 142.6(5)
(Point line) Fe (AF) 5.82 7.02 144.5(3) 119.1(2) 142.4(3)
M(2)-O-P -O-M(3) Mo (F) 5.66 7.05 140.2(5) 116.1(5) 139.2(7)
(Dashed line) Fe (AF) 5.55 6.91 138.0(3) 118.1(2) 138.8(3)
M(2)-O-P -O-M(3) Mo (F) 5.89 7.03 144.8(7) 118.9(5) 144.4(5)
(Dashed line) Fe (AF) 5.82 6.92 140.5(3) 115.5(2) 150.1(3)
M(3)-O-P -O-M’(3) Mo (F) 5.97 7.25 140.6(6) 112.1(5) 147.2(5)
(Dashed line) Fe (AF) 5.80 6.93 144.9(3) 119.6(3) 146.9(3)
Cr2(P6O18)
Cr-O(4)-P(1)-O(5)-Cr 5.36 6.86 147.9(9) 117.9(5) 137.1(8)
Cr-O(6)-P(2)-O(7)-Cr 6.19 6.86 137(1) 117.1(8) 139.2(9)
Cr-O(8)-P(3)-O(9)-Cr 5.01 6.87 135.3(8) 117.3(6) 132.3(7)
As can be deduced all contributions favor weak antiferromagnetic interactions. The Cr-O-
P-O-Cr angles being the origin of the overall magnetic behavior, do not show any trend in
Cr(PO3)3. These values are similar to those observed for Mo(PO3)3. A view of the most
important exchange pathways in both iron and molybdenum metaphosphates is given in
Figure 31. Solid lines represent connection between (100) layers in the [101] direction (Cc
setting) with always antiferromagnetic interactions (M= Mo, Fe). The values of the bond
distances and angles in the exchange pathways of the chromium and molybdenum phases are
clearly different from those of the Fe(PO3)3 where small magnetic frustration in the (100)
layers were observed [15]. This frustration in the layers of Fe(PO3)3 resulting from the
competition between super-superexchange interactions due to the connectivity in the (100)
layers are also present (see Figure 31). This fact could explain the presence of a weak
ferromagnetic component as was observed from magnetic measurements. These results
confirm than the different (MO6) octahedral geometry together with the electronic
configuration of the metal ions are responsible for the magnetic behavior observed in the
M(PO3)3 (M= Cr, Mo, Fe) metaphosphates.
In the case of B-type Cr2(P6O18) [see Figure 30(b)], the presence of one and two
exchange magnetic pathways is also observed. However, the Cr(III) ions are equivalent with a
centrosymmetric element simplifying the magnetic pathways in this phase. The Cr-O-P-O-Cr