How to center icon in a MaterialButton which has no text?

Found it. The attribute I was missing was app:iconPadding="0dp".

So, from my experiments, the minimum attributes needed to create a square MaterialButton which has a centred icon and no text is the following:

<com.google.android.material.button.MaterialButton
    android:layout_width="52dp"
    android:layout_height="52dp"
    android:insetLeft="0dp"
    android:insetTop="0dp"
    android:insetRight="0dp"
    android:insetBottom="0dp"
    app:icon="@drawable/icon_next"
    app:iconGravity="textStart"
    app:iconPadding="0dp" />

These attributes produce a MaterialButton as follows: