elasticity matrix elements order

Dear All,

I am trying to specify a new piezoelectric material. I have the all the required parameters, but I am uncertain about the order of the matrix elements. All matrices (e.g. elasticity matrix) are specified as a series of 21 numbers. I assume these are the upper triangular matrix elements of the corresponding 6x6 matrix. However, what is the elements order, is it row-wise or column wise?

Thanks, Silviu.